diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 50a62a50..1dc86d87 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -70,6 +70,56 @@ jobs:
         env:
           RUST_BACKTRACE: full
 
+  visual-tests:
+    needs:
+      - format
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v3
+        with:
+          submodules: recursive
+      - name: Get date for registry cache
+        id: date
+        run: echo "::set-output name=date::$(date +'%Y-%m-%d')"
+      - name: Cargo registry cache
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/registry/index
+            ~/.cargo/registry/cache
+            ~/.cargo/git
+          key: ${{ runner.os }}-cargo-registry-visual-${{ steps.date.outputs.date }}
+          restore-keys: ${{ runner.os }}-cargo-registry-visual-
+      - name: System dependencies
+        run: sudo apt-get update; sudo apt-get install -y libxkbcommon-dev libwayland-dev
+      - name: Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+      - name: Run visual snapshot tests
+        run: cargo test --no-default-features --test visual_tests
+        env:
+          RUST_BACKTRACE: full
+      - name: Run theme consistency tests
+        run: cargo test --no-default-features --test theme_tests
+        env:
+          RUST_BACKTRACE: full
+      - name: Run property-based tests
+        run: cargo test --no-default-features --test property_tests
+        env:
+          RUST_BACKTRACE: full
+      - name: Run widget unit tests
+        run: cargo test --no-default-features --test widget_unit_tests
+        env:
+          RUST_BACKTRACE: full
+      - name: Upload snapshot artifacts on failure
+        if: failure()
+        uses: actions/upload-artifact@v3
+        with:
+          name: snapshot-diffs
+          path: |
+            tests/snapshots/*.actual.png
+            tests/snapshots/*.diff.png
+
   examples:
     needs:
       - format
diff --git a/Cargo.toml b/Cargo.toml
index feaa8c74..dc0bdccf 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -226,3 +226,7 @@ dirs = "6.0.0"
 
 [dev-dependencies]
 tempfile = "3.24.0"
+proptest = "1.6"
+image = { version = "0.25.9", default-features = false, features = ["png"] }
+tiny-skia = "0.11"
+ron = "0.12"
diff --git a/src/widget/spin_button.rs b/src/widget/spin_button.rs
index 9ad81b4d..ad1042dc 100644
--- a/src/widget/spin_button.rs
+++ b/src/widget/spin_button.rs
@@ -318,8 +318,98 @@ fn container_style(theme: &crate::Theme) -> iced_widget::container::Style {
 
 #[cfg(test)]
 mod tests {
+    use super::*;
+
     #[test]
-    fn decrement() {
-        assert_eq!(super::decrement(0i32, 10, 15, 35), 15);
+    fn decrement_clamps_to_min() {
+        assert_eq!(decrement(0i32, 10, 15, 35), 15);
+    }
+
+    #[test]
+    fn decrement_normal_step() {
+        assert_eq!(decrement(20i32, 5, 0, 100), 15);
+    }
+
+    #[test]
+    fn decrement_at_min_stays_at_min() {
+        assert_eq!(decrement(0i32, 1, 0, 100), 0);
+    }
+
+    #[test]
+    fn decrement_near_min_clamps() {
+        assert_eq!(decrement(3i32, 5, 0, 100), 0);
+    }
+
+    #[test]
+    fn increment_normal_step() {
+        assert_eq!(increment(20i32, 5, 0, 100), 25);
+    }
+
+    #[test]
+    fn increment_clamps_to_max() {
+        assert_eq!(increment(98i32, 5, 0, 100), 100);
+    }
+
+    #[test]
+    fn increment_at_max_stays_at_max() {
+        assert_eq!(increment(100i32, 1, 0, 100), 100);
+    }
+
+    #[test]
+    fn increment_near_max_clamps() {
+        assert_eq!(increment(97i32, 5, 0, 100), 100);
+    }
+
+    #[test]
+    fn increment_then_decrement_round_trips() {
+        let val = 50i32;
+        let step = 10;
+        let min = 0;
+        let max = 100;
+        let up = increment(val, step, min, max);
+        let back = decrement(up, step, min, max);
+        assert_eq!(back, val);
+    }
+
+    #[test]
+    fn float_increment() {
+        let result = increment(0.5f64, 0.1, 0.0, 1.0);
+        assert!((result - 0.6).abs() < f64::EPSILON);
+    }
+
+    #[test]
+    fn float_decrement() {
+        let result = decrement(0.5f64, 0.1, 0.0, 1.0);
+        assert!((result - 0.4).abs() < f64::EPSILON);
+    }
+
+    #[test]
+    fn new_clamps_value_below_min() {
+        let sb = SpinButton::new("test", -5i32, 1, 0, 100, Orientation::Horizontal, |v| v);
+        assert_eq!(sb.value, 0);
+    }
+
+    #[test]
+    fn new_clamps_value_above_max() {
+        let sb = SpinButton::new("test", 200i32, 1, 0, 100, Orientation::Horizontal, |v| v);
+        assert_eq!(sb.value, 100);
+    }
+
+    #[test]
+    fn new_accepts_value_in_range() {
+        let sb = SpinButton::new("test", 50i32, 1, 0, 100, Orientation::Horizontal, |v| v);
+        assert_eq!(sb.value, 50);
+    }
+
+    #[test]
+    fn new_accepts_value_at_min() {
+        let sb = SpinButton::new("test", 0i32, 1, 0, 100, Orientation::Horizontal, |v| v);
+        assert_eq!(sb.value, 0);
+    }
+
+    #[test]
+    fn new_accepts_value_at_max() {
+        let sb = SpinButton::new("test", 100i32, 1, 0, 100, Orientation::Horizontal, |v| v);
+        assert_eq!(sb.value, 100);
     }
 }
diff --git a/tests/property_tests.rs b/tests/property_tests.rs
new file mode 100644
index 00000000..3ba74e1a
--- /dev/null
+++ b/tests/property_tests.rs
@@ -0,0 +1,258 @@
+// Copyright 2024 System76 <info@system76.com>
+// SPDX-License-Identifier: MPL-2.0
+
+//! Property-based tests for COSMIC data models and utility functions.
+//!
+//! Uses proptest to generate random inputs and verify that invariants hold
+//! across a wide range of values. This catches edge cases that hand-written
+//! unit tests often miss.
+
+use proptest::prelude::*;
+
+// ---------------------------------------------------------------------------
+// Spin button property tests
+// ---------------------------------------------------------------------------
+mod spin_button_props {
+    use super::*;
+
+    // We can't directly test the private increment/decrement functions,
+    // but we can replicate their logic and verify the same invariants.
+
+    fn increment(value: i32, step: i32, _min: i32, max: i32) -> i32 {
+        if value > max - step {
+            max
+        } else {
+            value + step
+        }
+    }
+
+    fn decrement(value: i32, step: i32, min: i32, _max: i32) -> i32 {
+        if value < min + step {
+            min
+        } else {
+            value - step
+        }
+    }
+
+    proptest! {
+        #[test]
+        fn increment_never_exceeds_max(
+            value in -1000i32..1000,
+            step in 1i32..100,
+            max in 0i32..2000,
+        ) {
+            let min = -1000;
+            let result = increment(value, step, min, max);
+            prop_assert!(result <= max, "increment({value}, {step}) = {result} > max({max})");
+        }
+
+        #[test]
+        fn decrement_never_goes_below_min(
+            value in -1000i32..1000,
+            step in 1i32..100,
+            min in -2000i32..0,
+        ) {
+            let max = 1000;
+            let result = decrement(value, step, min, max);
+            prop_assert!(result >= min, "decrement({value}, {step}) = {result} < min({min})");
+        }
+
+        #[test]
+        fn increment_at_max_stays_at_max(
+            step in 1i32..100,
+            max in 0i32..1000,
+        ) {
+            let min = -1000;
+            let result = increment(max, step, min, max);
+            prop_assert_eq!(result, max);
+        }
+
+        #[test]
+        fn decrement_at_min_stays_at_min(
+            step in 1i32..100,
+            min in -1000i32..0,
+        ) {
+            let max = 1000;
+            let result = decrement(min, step, min, max);
+            prop_assert_eq!(result, min);
+        }
+
+        #[test]
+        fn increment_then_decrement_returns_to_original_or_clamped(
+            value in -500i32..500,
+            step in 1i32..50,
+        ) {
+            let min = -1000;
+            let max = 1000;
+            let incremented = increment(value, step, min, max);
+            let round_tripped = decrement(incremented, step, min, max);
+            // If we didn't hit the max, we should return to original
+            if value <= max - step {
+                prop_assert_eq!(round_tripped, value);
+            } else {
+                prop_assert!(round_tripped >= value || round_tripped >= min);
+            }
+        }
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Calendar property tests
+// ---------------------------------------------------------------------------
+mod calendar_props {
+    use super::*;
+    use chrono::{Datelike, NaiveDate, Weekday};
+    use cosmic::widget::calendar::{get_calender_first, set_day};
+
+    proptest! {
+        #[test]
+        fn get_calendar_first_returns_correct_weekday(
+            year in 2000i32..2100,
+            month in 1u32..=12,
+            weekday_idx in 0u32..7,
+        ) {
+            let weekday = match weekday_idx {
+                0 => Weekday::Mon,
+                1 => Weekday::Tue,
+                2 => Weekday::Wed,
+                3 => Weekday::Thu,
+                4 => Weekday::Fri,
+                5 => Weekday::Sat,
+                _ => Weekday::Sun,
+            };
+            let first = get_calender_first(year, month, weekday);
+            prop_assert_eq!(first.weekday(), weekday);
+        }
+
+        #[test]
+        fn get_calendar_first_is_before_or_at_month_start(
+            year in 2000i32..2100,
+            month in 1u32..=12,
+        ) {
+            let first = get_calender_first(year, month, Weekday::Mon);
+            let month_start = NaiveDate::from_ymd_opt(year, month, 1).unwrap();
+            prop_assert!(first <= month_start,
+                "Calendar first {first} is after month start {month_start}");
+        }
+
+        #[test]
+        fn get_calendar_first_is_within_one_week_of_month_start(
+            year in 2000i32..2100,
+            month in 1u32..=12,
+        ) {
+            let first = get_calender_first(year, month, Weekday::Mon);
+            let month_start = NaiveDate::from_ymd_opt(year, month, 1).unwrap();
+            let diff = (month_start - first).num_days();
+            prop_assert!(diff >= 0 && diff < 7,
+                "Calendar first {first} is {diff} days before month start {month_start}");
+        }
+
+        #[test]
+        fn set_day_preserves_month_and_year(
+            month in 1u32..=12,
+            from_day in 1u32..=28,
+            to_day in 1u32..=28,
+        ) {
+            let date = NaiveDate::from_ymd_opt(2024, month, from_day).unwrap();
+            let result = set_day(date, to_day);
+            prop_assert_eq!(result.month(), month);
+            prop_assert_eq!(result.year(), 2024);
+        }
+
+        #[test]
+        fn set_day_idempotent(
+            month in 1u32..=12,
+            day in 1u32..=28,
+        ) {
+            let date = NaiveDate::from_ymd_opt(2024, month, day).unwrap();
+            let result = set_day(date, day);
+            prop_assert_eq!(result, date);
+        }
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Segmented button model property tests
+// ---------------------------------------------------------------------------
+mod segmented_model_props {
+    use super::*;
+    use cosmic::widget::segmented_button::{Model, SingleSelect, MultiSelect};
+
+    proptest! {
+        #[test]
+        fn model_length_matches_insertions(count in 0usize..50) {
+            let mut builder = Model::<SingleSelect>::builder();
+            for i in 0..count {
+                builder = builder.insert(move |b| b.text(format!("Item {i}")));
+            }
+            let model = builder.build();
+            prop_assert_eq!(model.len(), count);
+        }
+
+        #[test]
+        fn clear_always_empties_model(count in 1usize..20) {
+            let mut builder = Model::<SingleSelect>::builder();
+            for i in 0..count {
+                builder = builder.insert(move |b| b.text(format!("Item {i}")));
+            }
+            let mut model = builder.build();
+            model.clear();
+            prop_assert_eq!(model.len(), 0);
+        }
+
+        #[test]
+        fn multi_select_activate_is_toggle(count in 2usize..10) {
+            let mut builder = Model::<MultiSelect>::builder();
+            for i in 0..count {
+                builder = builder.insert(move |b| b.text(format!("Item {i}")));
+            }
+            let mut model = builder.build();
+
+            // Collect entities from the model
+            let ids: Vec<_> = model.iter().collect();
+
+            // Activate first item
+            model.activate(ids[0]);
+            prop_assert!(model.is_active(ids[0]));
+
+            // Toggle it off
+            model.activate(ids[0]);
+            prop_assert!(!model.is_active(ids[0]));
+        }
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Theme spacing property tests
+// ---------------------------------------------------------------------------
+mod theme_props {
+    use super::*;
+
+    #[test]
+    fn spacing_values_monotonically_increase() {
+        let spacing = cosmic::theme::spacing();
+        // xxs < xs < s < m < l < xl < xxl
+        assert!(spacing.space_xxs <= spacing.space_xs);
+        assert!(spacing.space_xs <= spacing.space_s);
+        assert!(spacing.space_s <= spacing.space_m);
+        assert!(spacing.space_m <= spacing.space_l);
+        assert!(spacing.space_l <= spacing.space_xl);
+        assert!(spacing.space_xl <= spacing.space_xxl);
+    }
+
+    proptest! {
+        #[test]
+        fn cosmic_theme_dark_default_is_valid(_dummy in 0..1u8) {
+            let theme = cosmic_theme::Theme::dark_default();
+            prop_assert!(theme.is_dark);
+            prop_assert!(theme.spacing.space_xxs > 0);
+        }
+
+        #[test]
+        fn cosmic_theme_light_default_is_valid(_dummy in 0..1u8) {
+            let theme = cosmic_theme::Theme::light_default();
+            prop_assert!(!theme.is_dark);
+            prop_assert!(theme.spacing.space_xxs > 0);
+        }
+    }
+}
diff --git a/tests/snapshot_harness/mod.rs b/tests/snapshot_harness/mod.rs
new file mode 100644
index 00000000..d4e11b35
--- /dev/null
+++ b/tests/snapshot_harness/mod.rs
@@ -0,0 +1,355 @@
+// Copyright 2024 System76 <info@system76.com>
+// SPDX-License-Identifier: MPL-2.0
+
+//! Snapshot testing harness for COSMIC widgets.
+//!
+//! Renders widgets to pixel buffers using the tiny-skia software renderer
+//! and compares against stored reference images. Provides deterministic,
+//! headless visual regression testing without requiring a display server.
+
+use cosmic::iced_core::{self, Rectangle, Size, layout, mouse, renderer};
+use cosmic::iced_core::widget::Tree;
+
+use std::path::{Path, PathBuf};
+
+/// Maximum per-pixel color channel difference allowed before a mismatch
+/// is flagged. Accounts for minor anti-aliasing and rounding differences.
+const PIXEL_TOLERANCE: u8 = 2;
+
+/// Maximum fraction of pixels allowed to differ before the snapshot is
+/// considered a failure (0.01 = 1%).
+const MISMATCH_THRESHOLD: f64 = 0.01;
+
+/// Test harness for rendering COSMIC widgets to pixel buffers.
+pub struct SnapshotHarness {
+    width: u32,
+    height: u32,
+}
+
+impl SnapshotHarness {
+    /// Create a new harness with the given pixel dimensions.
+    pub fn new(width: u32, height: u32) -> Self {
+        Self { width, height }
+    }
+
+    /// Render an element and compare it against a stored reference snapshot.
+    ///
+    /// If no reference exists, the rendered image is saved as the new reference.
+    /// If a reference exists, the rendered image is compared pixel-by-pixel.
+    pub fn snapshot<'a>(
+        &self,
+        name: &str,
+        element: cosmic::Element<'a, ()>,
+    ) {
+        let pixels = self.render_element(element);
+        let snapshot_dir = snapshot_dir();
+
+        std::fs::create_dir_all(&snapshot_dir)
+            .expect("Failed to create snapshots directory");
+
+        let reference_path = snapshot_dir.join(format!("{name}.png"));
+        let actual_path = snapshot_dir.join(format!("{name}.actual.png"));
+        let diff_path = snapshot_dir.join(format!("{name}.diff.png"));
+
+        // Save the current render
+        save_rgba_png(&actual_path, &pixels, self.width, self.height);
+
+        if reference_path.exists() {
+            // Compare against reference
+            let reference = load_png_rgba(&reference_path);
+            let result = compare_images(
+                &reference.data,
+                reference.width,
+                reference.height,
+                &pixels,
+                self.width,
+                self.height,
+            );
+
+            match result {
+                CompareResult::Match => {
+                    // Clean up actual file on success
+                    let _ = std::fs::remove_file(&actual_path);
+                    let _ = std::fs::remove_file(&diff_path);
+                }
+                CompareResult::SizeMismatch {
+                    ref_w,
+                    ref_h,
+                    act_w,
+                    act_h,
+                } => {
+                    panic!(
+                        "Snapshot '{name}' size mismatch: reference is {ref_w}x{ref_h}, \
+                         actual is {act_w}x{act_h}. \
+                         Actual saved to: {}\n\
+                         To update, delete the reference and re-run.",
+                        actual_path.display()
+                    );
+                }
+                CompareResult::PixelMismatch {
+                    mismatch_count,
+                    total_pixels,
+                    diff_image,
+                } => {
+                    let pct = (mismatch_count as f64 / total_pixels as f64) * 100.0;
+                    save_rgba_png(&diff_path, &diff_image, self.width, self.height);
+                    panic!(
+                        "Snapshot '{name}' has {mismatch_count}/{total_pixels} pixels \
+                         different ({pct:.2}%). Threshold: {:.2}%\n\
+                         Actual: {}\n\
+                         Diff:   {}\n\
+                         To update, delete the reference and re-run.",
+                        MISMATCH_THRESHOLD * 100.0,
+                        actual_path.display(),
+                        diff_path.display(),
+                    );
+                }
+            }
+        } else {
+            // No reference exists - promote actual to reference
+            std::fs::rename(&actual_path, &reference_path).expect("Failed to save reference");
+            eprintln!(
+                "Created new snapshot reference: {}",
+                reference_path.display()
+            );
+        }
+    }
+
+    /// Render a COSMIC element to an RGBA pixel buffer.
+    fn render_element<'a>(&self, mut element: cosmic::Element<'a, ()>) -> Vec<u8> {
+        let w = self.width as f32;
+        let h = self.height as f32;
+
+        // Build the widget tree
+        let mut tree = Tree::new(element.as_widget());
+
+        // Create a tiny-skia renderer
+        let mut renderer = iced_tiny_skia::Renderer::new(
+            iced_core::Font::DEFAULT,
+            iced_core::Pixels(14.0),
+        );
+
+        // Compute layout
+        let limits = layout::Limits::new(Size::ZERO, Size::new(w, h));
+        let node = element.as_widget_mut().layout(&mut tree, &renderer, &limits);
+        let layout_obj = layout::Layout::new(&node);
+
+        // Get theme
+        let cosmic_theme = cosmic::theme::active();
+
+        // Draw widget
+        let viewport = Rectangle {
+            x: 0.0,
+            y: 0.0,
+            width: w,
+            height: h,
+        };
+
+        let style = renderer::Style {
+            text_color: iced_core::Color::WHITE,
+            icon_color: iced_core::Color::WHITE,
+            scale_factor: 1.0,
+        };
+
+        element.as_widget().draw(
+            &tree,
+            &mut renderer,
+            &cosmic_theme,
+            &style,
+            layout_obj,
+            mouse::Cursor::Unavailable,
+            &viewport,
+        );
+
+        // Render to pixel buffer
+        let phys_w = self.width;
+        let phys_h = self.height;
+        let mut pixmap = tiny_skia::Pixmap::new(phys_w, phys_h)
+            .expect("Failed to create pixmap");
+
+        let viewport_obj = iced_tiny_skia::graphics::Viewport::with_logical_size(
+            Size::new(w, h),
+            1.0,
+        );
+
+        let mut mask = tiny_skia::Mask::new(phys_w, phys_h)
+            .expect("Failed to create mask");
+
+        let damage = vec![Rectangle {
+            x: 0.0,
+            y: 0.0,
+            width: w,
+            height: h,
+        }];
+
+        let bg_color = iced_core::Color::from_rgb(0.15, 0.15, 0.15);
+        let overlay: &[String] = &[];
+
+        renderer.draw(
+            &mut pixmap.as_mut(),
+            &mut mask,
+            &viewport_obj,
+            &damage,
+            bg_color,
+            overlay,
+        );
+
+        pixmap.data().to_vec()
+    }
+}
+
+/// Directory where snapshot reference images are stored.
+fn snapshot_dir() -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("tests").join("snapshots")
+}
+
+/// Save RGBA pixel data as a PNG file.
+fn save_rgba_png(path: &Path, data: &[u8], width: u32, height: u32) {
+    let img = image::RgbaImage::from_raw(width, height, data.to_vec())
+        .expect("Invalid image dimensions");
+    img.save(path)
+        .unwrap_or_else(|e| panic!("Failed to save PNG to {}: {e}", path.display()));
+}
+
+/// Loaded PNG image data.
+struct PngImage {
+    data: Vec<u8>,
+    width: u32,
+    height: u32,
+}
+
+/// Load a PNG file as RGBA pixel data.
+fn load_png_rgba(path: &Path) -> PngImage {
+    let img = image::open(path)
+        .unwrap_or_else(|e| panic!("Failed to load PNG from {}: {e}", path.display()))
+        .to_rgba8();
+    let width = img.width();
+    let height = img.height();
+    PngImage {
+        data: img.into_raw(),
+        width,
+        height,
+    }
+}
+
+/// Result of comparing two images.
+enum CompareResult {
+    Match,
+    SizeMismatch {
+        ref_w: u32,
+        ref_h: u32,
+        act_w: u32,
+        act_h: u32,
+    },
+    PixelMismatch {
+        mismatch_count: usize,
+        total_pixels: usize,
+        diff_image: Vec<u8>,
+    },
+}
+
+/// Compare two RGBA images pixel-by-pixel.
+fn compare_images(
+    reference: &[u8],
+    ref_w: u32,
+    ref_h: u32,
+    actual: &[u8],
+    act_w: u32,
+    act_h: u32,
+) -> CompareResult {
+    if ref_w != act_w || ref_h != act_h {
+        return CompareResult::SizeMismatch {
+            ref_w,
+            ref_h,
+            act_w,
+            act_h,
+        };
+    }
+
+    let total_pixels = (ref_w * ref_h) as usize;
+    let mut mismatch_count = 0;
+    let mut diff_image = vec![0u8; reference.len()];
+
+    for i in 0..total_pixels {
+        let offset = i * 4;
+        let r_diff = (reference[offset] as i16 - actual[offset] as i16).unsigned_abs() as u8;
+        let g_diff =
+            (reference[offset + 1] as i16 - actual[offset + 1] as i16).unsigned_abs() as u8;
+        let b_diff =
+            (reference[offset + 2] as i16 - actual[offset + 2] as i16).unsigned_abs() as u8;
+        let a_diff =
+            (reference[offset + 3] as i16 - actual[offset + 3] as i16).unsigned_abs() as u8;
+
+        if r_diff > PIXEL_TOLERANCE
+            || g_diff > PIXEL_TOLERANCE
+            || b_diff > PIXEL_TOLERANCE
+            || a_diff > PIXEL_TOLERANCE
+        {
+            mismatch_count += 1;
+            // Highlight differences in red
+            diff_image[offset] = 255;
+            diff_image[offset + 1] = 0;
+            diff_image[offset + 2] = 0;
+            diff_image[offset + 3] = 255;
+        } else {
+            // Dim matching pixels
+            diff_image[offset] = actual[offset] / 3;
+            diff_image[offset + 1] = actual[offset + 1] / 3;
+            diff_image[offset + 2] = actual[offset + 2] / 3;
+            diff_image[offset + 3] = 255;
+        }
+    }
+
+    let mismatch_fraction = mismatch_count as f64 / total_pixels as f64;
+    if mismatch_fraction <= MISMATCH_THRESHOLD {
+        CompareResult::Match
+    } else {
+        CompareResult::PixelMismatch {
+            mismatch_count,
+            total_pixels,
+            diff_image,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn compare_identical_images() {
+        let data = vec![128u8; 4 * 10 * 10];
+        let result = compare_images(&data, 10, 10, &data, 10, 10);
+        assert!(matches!(result, CompareResult::Match));
+    }
+
+    #[test]
+    fn compare_different_sizes() {
+        let small = vec![0u8; 4 * 5 * 5];
+        let big = vec![0u8; 4 * 10 * 10];
+        let result = compare_images(&small, 5, 5, &big, 10, 10);
+        assert!(matches!(result, CompareResult::SizeMismatch { .. }));
+    }
+
+    #[test]
+    fn compare_within_tolerance() {
+        let a = vec![100u8; 4 * 4 * 4];
+        let mut b = a.clone();
+        // Change one channel by 1 (within tolerance of 2)
+        b[0] = 101;
+        let result = compare_images(&a, 4, 4, &b, 4, 4);
+        assert!(matches!(result, CompareResult::Match));
+    }
+
+    #[test]
+    fn compare_exceeds_tolerance() {
+        let a = vec![100u8; 4 * 2 * 2]; // 4 pixels
+        let mut b = a.clone();
+        // Make all pixels differ significantly
+        for i in (0..b.len()).step_by(4) {
+            b[i] = 200;
+        }
+        let result = compare_images(&a, 2, 2, &b, 2, 2);
+        assert!(matches!(result, CompareResult::PixelMismatch { .. }));
+    }
+}
diff --git a/tests/snapshots/column_layout.png b/tests/snapshots/column_layout.png
new file mode 100644
index 00000000..1a3026e8
Binary files /dev/null and b/tests/snapshots/column_layout.png differ
diff --git a/tests/snapshots/container_with_background.png b/tests/snapshots/container_with_background.png
new file mode 100644
index 00000000..02e5f535
Binary files /dev/null and b/tests/snapshots/container_with_background.png differ
diff --git a/tests/snapshots/dark_theme_container.png b/tests/snapshots/dark_theme_container.png
new file mode 100644
index 00000000..b00c56a3
Binary files /dev/null and b/tests/snapshots/dark_theme_container.png differ
diff --git a/tests/snapshots/divider_heavy.png b/tests/snapshots/divider_heavy.png
new file mode 100644
index 00000000..8b6a1666
Binary files /dev/null and b/tests/snapshots/divider_heavy.png differ
diff --git a/tests/snapshots/divider_horizontal.png b/tests/snapshots/divider_horizontal.png
new file mode 100644
index 00000000..e0910fef
Binary files /dev/null and b/tests/snapshots/divider_horizontal.png differ
diff --git a/tests/snapshots/nested_containers.png b/tests/snapshots/nested_containers.png
new file mode 100644
index 00000000..7d0ad195
Binary files /dev/null and b/tests/snapshots/nested_containers.png differ
diff --git a/tests/snapshots/progress_bar_states.png b/tests/snapshots/progress_bar_states.png
new file mode 100644
index 00000000..03f77d2f
Binary files /dev/null and b/tests/snapshots/progress_bar_states.png differ
diff --git a/tests/snapshots/row_layout.png b/tests/snapshots/row_layout.png
new file mode 100644
index 00000000..43a8471e
Binary files /dev/null and b/tests/snapshots/row_layout.png differ
diff --git a/tests/snapshots/text_styles.png b/tests/snapshots/text_styles.png
new file mode 100644
index 00000000..683f5b9a
Binary files /dev/null and b/tests/snapshots/text_styles.png differ
diff --git a/tests/theme_tests.rs b/tests/theme_tests.rs
new file mode 100644
index 00000000..d34dc0f1
--- /dev/null
+++ b/tests/theme_tests.rs
@@ -0,0 +1,291 @@
+// Copyright 2024 System76 <info@system76.com>
+// SPDX-License-Identifier: MPL-2.0
+
+//! Theme and style consistency tests for COSMIC.
+//!
+//! These tests verify that the COSMIC theme system produces consistent,
+//! valid output across different theme variants (light, dark, high-contrast).
+//! They catch issues like missing colors, zero-alpha components, and
+//! inconsistencies between theme variants.
+
+use cosmic::theme;
+use cosmic_theme::Theme as CosmicTheme;
+
+// ---------------------------------------------------------------------------
+// Theme variant consistency tests
+// ---------------------------------------------------------------------------
+
+fn all_themes() -> Vec<(&'static str, &'static CosmicTheme)> {
+    vec![
+        ("dark", &*theme::COSMIC_DARK),
+        ("light", &*theme::COSMIC_LIGHT),
+        ("hc_dark", &*theme::COSMIC_HC_DARK),
+        ("hc_light", &*theme::COSMIC_HC_LIGHT),
+    ]
+}
+
+#[test]
+fn all_themes_have_valid_spacing() {
+    for (name, t) in all_themes() {
+        assert!(t.spacing.space_xxs > 0, "{name}: space_xxs is zero");
+        assert!(t.spacing.space_xs > 0, "{name}: space_xs is zero");
+        assert!(t.spacing.space_s > 0, "{name}: space_s is zero");
+        assert!(t.spacing.space_m > 0, "{name}: space_m is zero");
+        assert!(t.spacing.space_l > 0, "{name}: space_l is zero");
+        assert!(t.spacing.space_xl > 0, "{name}: space_xl is zero");
+        assert!(t.spacing.space_xxl > 0, "{name}: space_xxl is zero");
+    }
+}
+
+#[test]
+fn all_themes_have_valid_corner_radii() {
+    for (name, t) in all_themes() {
+        let radii = &t.corner_radii;
+        // Corner radii should be non-negative
+        for r in &radii.radius_0 {
+            assert!(*r >= 0.0, "{name}: negative radius_0");
+        }
+        for r in &radii.radius_xs {
+            assert!(*r >= 0.0, "{name}: negative radius_xs");
+        }
+        for r in &radii.radius_s {
+            assert!(*r >= 0.0, "{name}: negative radius_s");
+        }
+        for r in &radii.radius_m {
+            assert!(*r >= 0.0, "{name}: negative radius_m");
+        }
+        for r in &radii.radius_l {
+            assert!(*r >= 0.0, "{name}: negative radius_l");
+        }
+        for r in &radii.radius_xl {
+            assert!(*r >= 0.0, "{name}: negative radius_xl");
+        }
+    }
+}
+
+#[test]
+fn all_themes_have_non_zero_alpha_accent() {
+    for (name, t) in all_themes() {
+        let accent = &t.accent;
+        assert!(
+            accent.base.alpha > 0.0,
+            "{name}: accent base alpha is zero"
+        );
+    }
+}
+
+#[test]
+fn dark_themes_have_is_dark_set() {
+    assert!(theme::COSMIC_DARK.is_dark);
+    assert!(theme::COSMIC_HC_DARK.is_dark);
+}
+
+#[test]
+fn light_themes_have_is_dark_unset() {
+    assert!(!theme::COSMIC_LIGHT.is_dark);
+    assert!(!theme::COSMIC_HC_LIGHT.is_dark);
+}
+
+#[test]
+fn all_themes_have_non_zero_alpha_on_bg() {
+    for (name, t) in all_themes() {
+        let bg = &t.background;
+        assert!(
+            bg.base.alpha > 0.0,
+            "{name}: background base alpha is zero"
+        );
+    }
+}
+
+#[test]
+fn all_themes_have_non_zero_alpha_on_primary() {
+    for (name, t) in all_themes() {
+        let primary = &t.primary;
+        assert!(
+            primary.base.alpha > 0.0,
+            "{name}: primary base alpha is zero"
+        );
+    }
+}
+
+#[test]
+fn all_themes_have_non_zero_alpha_on_destructive() {
+    for (name, t) in all_themes() {
+        let dest = &t.destructive;
+        assert!(
+            dest.base.alpha > 0.0,
+            "{name}: destructive base alpha is zero"
+        );
+    }
+}
+
+#[test]
+fn all_themes_have_non_zero_alpha_on_warning() {
+    for (name, t) in all_themes() {
+        let warn = &t.warning;
+        assert!(
+            warn.base.alpha > 0.0,
+            "{name}: warning base alpha is zero"
+        );
+    }
+}
+
+#[test]
+fn all_themes_have_non_zero_alpha_on_success() {
+    for (name, t) in all_themes() {
+        let success = &t.success;
+        assert!(
+            success.base.alpha > 0.0,
+            "{name}: success base alpha is zero"
+        );
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Component state consistency tests
+// ---------------------------------------------------------------------------
+
+/// Verify that all states of a Component have non-zero alpha.
+fn assert_component_states_visible(name: &str, variant: &str, component: &cosmic_theme::Component) {
+    assert!(
+        component.base.alpha > 0.0,
+        "{name}/{variant}: base alpha is zero"
+    );
+    // hover/pressed/selected states can have zero alpha in some themes, but
+    // the 'on' color (text on top) should always be visible
+    assert!(
+        component.on.alpha > 0.0,
+        "{name}/{variant}: 'on' (text) alpha is zero"
+    );
+}
+
+#[test]
+fn background_component_states_visible() {
+    for (name, t) in all_themes() {
+        assert_component_states_visible(name, "bg.component", &t.background.component);
+    }
+}
+
+#[test]
+fn primary_component_states_visible() {
+    for (name, t) in all_themes() {
+        assert_component_states_visible(name, "primary.component", &t.primary.component);
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Serialization round-trip test
+// ---------------------------------------------------------------------------
+
+#[test]
+fn dark_theme_serializes_to_ron() {
+    let theme = CosmicTheme::dark_default();
+    let serialized = ron::to_string(&theme);
+    assert!(serialized.is_ok(), "Dark theme failed to serialize to RON");
+}
+
+#[test]
+fn light_theme_serializes_to_ron() {
+    let theme = CosmicTheme::light_default();
+    let serialized = ron::to_string(&theme);
+    assert!(serialized.is_ok(), "Light theme failed to serialize to RON");
+}
+
+#[test]
+fn dark_theme_ron_round_trip() {
+    let theme = CosmicTheme::dark_default();
+    let serialized = ron::to_string(&theme).expect("serialize");
+    let deserialized: CosmicTheme = ron::from_str(&serialized).expect("deserialize");
+
+    assert_eq!(theme.is_dark, deserialized.is_dark);
+    assert_eq!(theme.spacing.space_m, deserialized.spacing.space_m);
+}
+
+#[test]
+fn light_theme_ron_round_trip() {
+    let theme = CosmicTheme::light_default();
+    let serialized = ron::to_string(&theme).expect("serialize");
+    let deserialized: CosmicTheme = ron::from_str(&serialized).expect("deserialize");
+
+    assert_eq!(theme.is_dark, deserialized.is_dark);
+    assert_eq!(theme.spacing.space_m, deserialized.spacing.space_m);
+}
+
+// ---------------------------------------------------------------------------
+// Spacing monotonicity
+// ---------------------------------------------------------------------------
+
+#[test]
+fn all_themes_spacing_is_monotonic() {
+    for (name, t) in all_themes() {
+        let s = &t.spacing;
+        assert!(
+            s.space_xxs <= s.space_xs,
+            "{name}: xxs({}) > xs({})",
+            s.space_xxs,
+            s.space_xs
+        );
+        assert!(
+            s.space_xs <= s.space_s,
+            "{name}: xs({}) > s({})",
+            s.space_xs,
+            s.space_s
+        );
+        assert!(
+            s.space_s <= s.space_m,
+            "{name}: s({}) > m({})",
+            s.space_s,
+            s.space_m
+        );
+        assert!(
+            s.space_m <= s.space_l,
+            "{name}: m({}) > l({})",
+            s.space_m,
+            s.space_l
+        );
+        assert!(
+            s.space_l <= s.space_xl,
+            "{name}: l({}) > xl({})",
+            s.space_l,
+            s.space_xl
+        );
+        assert!(
+            s.space_xl <= s.space_xxl,
+            "{name}: xl({}) > xxl({})",
+            s.space_xl,
+            s.space_xxl
+        );
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Corner radii ordering
+// ---------------------------------------------------------------------------
+
+#[test]
+fn all_themes_corner_radii_are_ordered() {
+    for (name, t) in all_themes() {
+        let r = &t.corner_radii;
+        // radius_0 should be all zeros
+        for val in &r.radius_0 {
+            assert_eq!(*val, 0.0, "{name}: radius_0 is not zero");
+        }
+        // Each subsequent radius should be >= the previous (at index 0)
+        assert!(
+            r.radius_xs[0] <= r.radius_s[0],
+            "{name}: radius_xs > radius_s"
+        );
+        assert!(
+            r.radius_s[0] <= r.radius_m[0],
+            "{name}: radius_s > radius_m"
+        );
+        assert!(
+            r.radius_m[0] <= r.radius_l[0],
+            "{name}: radius_m > radius_l"
+        );
+        assert!(
+            r.radius_l[0] <= r.radius_xl[0],
+            "{name}: radius_l > radius_xl"
+        );
+    }
+}
diff --git a/tests/visual_tests.rs b/tests/visual_tests.rs
new file mode 100644
index 00000000..a756e320
--- /dev/null
+++ b/tests/visual_tests.rs
@@ -0,0 +1,183 @@
+// Copyright 2024 System76 <info@system76.com>
+// SPDX-License-Identifier: MPL-2.0
+
+//! Visual snapshot tests for COSMIC widgets.
+//!
+//! These tests render widgets using the tiny-skia software renderer and compare
+//! the output against stored reference snapshots. This catches unintended visual
+//! regressions in widget rendering, theming, and layout.
+//!
+//! # Updating Snapshots
+//!
+//! When a visual change is intentional, delete the old snapshot PNG from
+//! `tests/snapshots/` and re-run the tests. The new reference will be generated
+//! automatically.
+
+mod snapshot_harness;
+
+use cosmic::iced_core::Length;
+use cosmic::theme;
+use snapshot_harness::SnapshotHarness;
+
+/// Helper to render a cosmic Element to a snapshot.
+fn render_element_snapshot(
+    name: &str,
+    width: u32,
+    height: u32,
+    element: cosmic::Element<'_, ()>,
+) {
+    let harness = SnapshotHarness::new(width, height);
+    harness.snapshot(name, element);
+}
+
+// ---------------------------------------------------------------------------
+// Theme rendering tests - verify that the cosmic theme system produces
+// consistent visual output for core UI primitives.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn snapshot_container_with_background() {
+    use cosmic::widget::{container, text};
+
+    let content = text::body("Hello, COSMIC!")
+        .apply(container)
+        .padding(16)
+        .width(Length::Fixed(200.0))
+        .height(Length::Fixed(60.0))
+        .class(theme::Container::Primary);
+
+    render_element_snapshot("container_with_background", 220, 80, content.into());
+}
+
+#[test]
+fn snapshot_divider_horizontal() {
+    use cosmic::widget::{column, container, divider};
+
+    let content = column::with_capacity(3)
+        .push(cosmic::widget::text::body("Above"))
+        .push(divider::horizontal::default())
+        .push(cosmic::widget::text::body("Below"))
+        .width(Length::Fixed(200.0))
+        .apply(container)
+        .padding(8);
+
+    render_element_snapshot("divider_horizontal", 220, 100, content.into());
+}
+
+#[test]
+fn snapshot_divider_heavy() {
+    use cosmic::widget::{column, container, divider};
+
+    let content = column::with_capacity(3)
+        .push(cosmic::widget::text::body("Above"))
+        .push(divider::horizontal::heavy())
+        .push(cosmic::widget::text::body("Below"))
+        .width(Length::Fixed(200.0))
+        .apply(container)
+        .padding(8);
+
+    render_element_snapshot("divider_heavy", 220, 100, content.into());
+}
+
+#[test]
+fn snapshot_text_styles() {
+    use cosmic::widget::{column, container, text};
+
+    let content = column::with_capacity(4)
+        .push(text::title3("Title Text"))
+        .push(text::body("Body text content"))
+        .push(text::caption("Caption text"))
+        .push(text::monotext("Monospace text"))
+        .spacing(8)
+        .width(Length::Fixed(300.0))
+        .apply(container)
+        .padding(16);
+
+    render_element_snapshot("text_styles", 340, 200, content.into());
+}
+
+#[test]
+fn snapshot_progress_bar() {
+    use cosmic::widget::{column, container, progress_bar};
+
+    let content = column::with_capacity(3)
+        .push(progress_bar(0.0..=100.0, 0.0).width(Length::Fixed(250.0)))
+        .push(progress_bar(0.0..=100.0, 50.0).width(Length::Fixed(250.0)))
+        .push(progress_bar(0.0..=100.0, 100.0).width(Length::Fixed(250.0)))
+        .spacing(12)
+        .apply(container)
+        .padding(16);
+
+    render_element_snapshot("progress_bar_states", 290, 120, content.into());
+}
+
+#[test]
+fn snapshot_row_layout() {
+    use cosmic::widget::{container, row, text};
+
+    let content = row::with_capacity(3)
+        .push(text::body("Left"))
+        .push(text::body("Center"))
+        .push(text::body("Right"))
+        .spacing(16)
+        .apply(container)
+        .padding(12);
+
+    render_element_snapshot("row_layout", 300, 60, content.into());
+}
+
+#[test]
+fn snapshot_column_layout() {
+    use cosmic::widget::{column, container, text};
+
+    let content = column::with_capacity(3)
+        .push(text::body("First"))
+        .push(text::body("Second"))
+        .push(text::body("Third"))
+        .spacing(8)
+        .apply(container)
+        .padding(12);
+
+    render_element_snapshot("column_layout", 200, 120, content.into());
+}
+
+#[test]
+fn snapshot_nested_containers() {
+    use cosmic::widget::{column, container, row, text};
+
+    let inner = column::with_capacity(2)
+        .push(text::body("Nested"))
+        .push(text::caption("Content"))
+        .spacing(4)
+        .apply(container)
+        .padding(8)
+        .class(theme::Container::Primary);
+
+    let outer = row::with_capacity(2)
+        .push(inner)
+        .push(text::body("Adjacent"))
+        .spacing(16)
+        .apply(container)
+        .padding(12);
+
+    render_element_snapshot("nested_containers", 300, 120, outer.into());
+}
+
+#[test]
+fn snapshot_dark_theme_container() {
+    use cosmic::widget::{container, text};
+
+    // Set dark theme temporarily for this test
+    let content = text::body("Dark Theme")
+        .apply(container)
+        .padding(16)
+        .width(Length::Fixed(200.0))
+        .height(Length::Fixed(60.0))
+        .class(theme::Container::Primary);
+
+    // The default theme is dark, so this tests the default rendering
+    render_element_snapshot("dark_theme_container", 220, 80, content.into());
+}
+
+// We use `apply` from the Apply trait for chaining
+use apply::Apply;
diff --git a/tests/widget_unit_tests.rs b/tests/widget_unit_tests.rs
new file mode 100644
index 00000000..6a8e1b09
--- /dev/null
+++ b/tests/widget_unit_tests.rs
@@ -0,0 +1,555 @@
+// Copyright 2024 System76 <info@system76.com>
+// SPDX-License-Identifier: MPL-2.0
+
+//! Unit tests for COSMIC widget logic and models.
+//!
+//! These tests verify the behavioral correctness of widget models,
+//! helper functions, and data structures without requiring a renderer.
+
+// ---------------------------------------------------------------------------
+// Spin button logic tests
+// ---------------------------------------------------------------------------
+mod spin_button_tests {
+    use cosmic::widget::spin_button;
+
+    // Test the internal increment/decrement functions via the public API.
+    // The functions are private, so we test through behavior.
+
+    #[test]
+    fn spin_button_clamps_value_to_min() {
+        // When value is below min, it should be clamped
+        let button = spin_button::spin_button(
+            "-5",
+            #[cfg(feature = "a11y")]
+            "test",
+            -5i32,
+            1,
+            0,
+            100,
+            |v| v,
+        );
+        // The SpinButton constructor clamps value to min when value < min
+        // We verify this through the element creation (it won't panic)
+        let _: cosmic::Element<'_, i32> = button.into();
+    }
+
+    #[test]
+    fn spin_button_clamps_value_to_max() {
+        let button = spin_button::spin_button(
+            "150",
+            #[cfg(feature = "a11y")]
+            "test",
+            150i32,
+            1,
+            0,
+            100,
+            |v| v,
+        );
+        let _: cosmic::Element<'_, i32> = button.into();
+    }
+
+    #[test]
+    fn spin_button_accepts_valid_range() {
+        let button = spin_button::spin_button(
+            "50",
+            #[cfg(feature = "a11y")]
+            "test",
+            50i32,
+            1,
+            0,
+            100,
+            |v| v,
+        );
+        let _: cosmic::Element<'_, i32> = button.into();
+    }
+
+    #[test]
+    fn spin_button_vertical_variant() {
+        let button = spin_button::vertical(
+            "25",
+            #[cfg(feature = "a11y")]
+            "vertical test",
+            25i32,
+            5,
+            0,
+            100,
+            |v| v,
+        );
+        let _: cosmic::Element<'_, i32> = button.into();
+    }
+
+    #[test]
+    fn spin_button_float_values() {
+        let button = spin_button::spin_button(
+            "3.14",
+            #[cfg(feature = "a11y")]
+            "float test",
+            3.14f64,
+            0.1,
+            0.0,
+            10.0,
+            |v| v,
+        );
+        let _: cosmic::Element<'_, f64> = button.into();
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Calendar model tests
+// ---------------------------------------------------------------------------
+mod calendar_model_tests {
+    use chrono::{Datelike, NaiveDate, Weekday};
+    use cosmic::widget::calendar::{CalendarModel, get_calender_first, set_day};
+
+    #[test]
+    fn calendar_model_creation() {
+        let model = CalendarModel::now();
+        assert_eq!(model.selected, model.visible);
+    }
+
+    #[test]
+    fn calendar_model_custom_date() {
+        let date = NaiveDate::from_ymd_opt(2024, 6, 15).unwrap();
+        let model = CalendarModel::new(date, date);
+        assert_eq!(model.selected.month(), 6);
+        assert_eq!(model.selected.day(), 15);
+    }
+
+    #[test]
+    fn calendar_show_prev_month() {
+        let date = NaiveDate::from_ymd_opt(2024, 3, 15).unwrap();
+        let mut model = CalendarModel::new(date, date);
+        model.show_prev_month();
+        assert_eq!(model.visible.month(), 2);
+        assert_eq!(model.selected, date); // selected unchanged
+    }
+
+    #[test]
+    fn calendar_show_next_month() {
+        let date = NaiveDate::from_ymd_opt(2024, 11, 15).unwrap();
+        let mut model = CalendarModel::new(date, date);
+        model.show_next_month();
+        assert_eq!(model.visible.month(), 12);
+    }
+
+    #[test]
+    fn calendar_show_prev_month_wraps_year() {
+        let date = NaiveDate::from_ymd_opt(2024, 1, 15).unwrap();
+        let mut model = CalendarModel::new(date, date);
+        model.show_prev_month();
+        assert_eq!(model.visible.month(), 12);
+        assert_eq!(model.visible.year(), 2023);
+    }
+
+    #[test]
+    fn calendar_show_next_month_wraps_year() {
+        let date = NaiveDate::from_ymd_opt(2024, 12, 15).unwrap();
+        let mut model = CalendarModel::new(date, date);
+        model.show_next_month();
+        assert_eq!(model.visible.month(), 1);
+        assert_eq!(model.visible.year(), 2025);
+    }
+
+    #[test]
+    fn calendar_set_prev_month_updates_selected() {
+        let date = NaiveDate::from_ymd_opt(2024, 6, 15).unwrap();
+        let mut model = CalendarModel::new(date, date);
+        model.set_prev_month();
+        assert_eq!(model.selected, model.visible);
+        assert_eq!(model.visible.month(), 5);
+    }
+
+    #[test]
+    fn calendar_set_next_month_updates_selected() {
+        let date = NaiveDate::from_ymd_opt(2024, 6, 15).unwrap();
+        let mut model = CalendarModel::new(date, date);
+        model.set_next_month();
+        assert_eq!(model.selected, model.visible);
+        assert_eq!(model.visible.month(), 7);
+    }
+
+    #[test]
+    fn calendar_set_selected_visible() {
+        let date = NaiveDate::from_ymd_opt(2024, 6, 15).unwrap();
+        let new_date = NaiveDate::from_ymd_opt(2025, 1, 1).unwrap();
+        let mut model = CalendarModel::new(date, date);
+        model.set_selected_visible(new_date);
+        assert_eq!(model.selected, new_date);
+        assert_eq!(model.visible, new_date);
+    }
+
+    #[test]
+    fn set_day_increases() {
+        let date = NaiveDate::from_ymd_opt(2024, 6, 10).unwrap();
+        let result = set_day(date, 20);
+        assert_eq!(result.day(), 20);
+        assert_eq!(result.month(), 6);
+    }
+
+    #[test]
+    fn set_day_decreases() {
+        let date = NaiveDate::from_ymd_opt(2024, 6, 20).unwrap();
+        let result = set_day(date, 5);
+        assert_eq!(result.day(), 5);
+    }
+
+    #[test]
+    fn set_day_same_returns_original() {
+        let date = NaiveDate::from_ymd_opt(2024, 6, 15).unwrap();
+        let result = set_day(date, 15);
+        assert_eq!(result, date);
+    }
+
+    #[test]
+    fn get_calendar_first_monday_start() {
+        // June 2024 starts on a Saturday
+        let first = get_calender_first(2024, 6, Weekday::Mon);
+        assert_eq!(first.weekday(), Weekday::Mon);
+        // The first Monday before June 1 (Saturday) is May 27
+        assert_eq!(first.day(), 27);
+        assert_eq!(first.month(), 5);
+    }
+
+    #[test]
+    fn get_calendar_first_sunday_start() {
+        let first = get_calender_first(2024, 6, Weekday::Sun);
+        assert_eq!(first.weekday(), Weekday::Sun);
+        assert_eq!(first.day(), 26);
+        assert_eq!(first.month(), 5);
+    }
+
+    #[test]
+    fn get_calendar_first_when_month_starts_on_first_day() {
+        // April 2024 starts on a Monday
+        let first = get_calender_first(2024, 4, Weekday::Mon);
+        assert_eq!(first.day(), 1);
+        assert_eq!(first.month(), 4);
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Segmented button model tests
+// ---------------------------------------------------------------------------
+mod segmented_model_tests {
+    use cosmic::widget::segmented_button::{Model, SingleSelect, MultiSelect};
+
+    #[test]
+    fn builder_creates_items_in_order() {
+        let model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("First"))
+            .insert(|b| b.text("Second"))
+            .insert(|b| b.text("Third"))
+            .build();
+
+        let texts: Vec<&str> = model
+            .iter()
+            .filter_map(|id| model.text(id))
+            .collect();
+
+        assert_eq!(texts, vec!["First", "Second", "Third"]);
+    }
+
+    #[test]
+    fn single_select_activates_only_one() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("A").with_id(|id| ids.push(id)))
+            .insert(|b| b.text("B").with_id(|id| ids.push(id)))
+            .insert(|b| b.text("C").with_id(|id| ids.push(id)))
+            .build();
+
+        model.activate(ids[0]);
+        assert!(model.is_active(ids[0]));
+        assert!(!model.is_active(ids[1]));
+
+        model.activate(ids[1]);
+        assert!(!model.is_active(ids[0]));
+        assert!(model.is_active(ids[1]));
+    }
+
+    #[test]
+    fn multi_select_toggles_items() {
+        let mut ids = Vec::new();
+        let mut model: Model<MultiSelect> = Model::builder()
+            .insert(|b| b.text("A").with_id(|id| ids.push(id)))
+            .insert(|b| b.text("B").with_id(|id| ids.push(id)))
+            .build();
+
+        model.activate(ids[0]);
+        model.activate(ids[1]);
+        assert!(model.is_active(ids[0]));
+        assert!(model.is_active(ids[1]));
+
+        // Toggle off
+        model.activate(ids[0]);
+        assert!(!model.is_active(ids[0]));
+        assert!(model.is_active(ids[1]));
+    }
+
+    #[test]
+    fn remove_item_reduces_length() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("A").with_id(|id| ids.push(id)))
+            .insert(|b| b.text("B").with_id(|id| ids.push(id)))
+            .insert(|b| b.text("C").with_id(|id| ids.push(id)))
+            .build();
+
+        assert_eq!(model.len(), 3);
+        model.remove(ids[1]);
+        assert_eq!(model.len(), 2);
+        assert!(!model.contains_item(ids[1]));
+    }
+
+    #[test]
+    fn clear_removes_all() {
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("A"))
+            .insert(|b| b.text("B"))
+            .build();
+
+        model.clear();
+        assert_eq!(model.len(), 0);
+    }
+
+    #[test]
+    fn data_storage_and_retrieval() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("Item").with_id(|id| ids.push(id)))
+            .build();
+
+        model.data_set::<u32>(ids[0], 42);
+        assert_eq!(model.data::<u32>(ids[0]), Some(&42));
+
+        model.data_remove::<u32>(ids[0]);
+        assert_eq!(model.data::<u32>(ids[0]), None);
+    }
+
+    #[test]
+    fn text_set_and_remove() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("Original").with_id(|id| ids.push(id)))
+            .build();
+
+        assert_eq!(model.text(ids[0]), Some("Original"));
+
+        model.text_set(ids[0], "Updated");
+        assert_eq!(model.text(ids[0]), Some("Updated"));
+
+        model.text_remove(ids[0]);
+        assert_eq!(model.text(ids[0]), None);
+    }
+
+    #[test]
+    fn enable_disable_items() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("Item").with_id(|id| ids.push(id)))
+            .build();
+
+        assert!(model.is_enabled(ids[0]));
+
+        model.enable(ids[0], false);
+        assert!(!model.is_enabled(ids[0]));
+
+        model.enable(ids[0], true);
+        assert!(model.is_enabled(ids[0]));
+    }
+
+    #[test]
+    fn position_operations() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("A").with_id(|id| ids.push(id)))
+            .insert(|b| b.text("B").with_id(|id| ids.push(id)))
+            .insert(|b| b.text("C").with_id(|id| ids.push(id)))
+            .build();
+
+        assert_eq!(model.position(ids[0]), Some(0));
+        assert_eq!(model.position(ids[1]), Some(1));
+        assert_eq!(model.position(ids[2]), Some(2));
+
+        // Move C to position 0
+        model.position_set(ids[2], 0);
+        assert_eq!(model.position(ids[2]), Some(0));
+        assert_eq!(model.position(ids[0]), Some(1));
+    }
+
+    #[test]
+    fn position_swap() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("A").with_id(|id| ids.push(id)))
+            .insert(|b| b.text("B").with_id(|id| ids.push(id)))
+            .build();
+
+        assert!(model.position_swap(ids[0], ids[1]));
+        assert_eq!(model.position(ids[0]), Some(1));
+        assert_eq!(model.position(ids[1]), Some(0));
+    }
+
+    #[test]
+    fn entity_at_returns_correct_entity() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("A").with_id(|id| ids.push(id)))
+            .insert(|b| b.text("B").with_id(|id| ids.push(id)))
+            .build();
+
+        assert_eq!(model.entity_at(0), Some(ids[0]));
+        assert_eq!(model.entity_at(1), Some(ids[1]));
+        assert_eq!(model.entity_at(2), None);
+    }
+
+    #[test]
+    fn activate_position_works() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("A").with_id(|id| ids.push(id)))
+            .insert(|b| b.text("B").with_id(|id| ids.push(id)))
+            .build();
+
+        assert!(model.activate_position(1));
+        assert!(model.is_active(ids[1]));
+
+        assert!(!model.activate_position(5)); // Out of bounds
+    }
+
+    #[test]
+    fn closable_set_and_check() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("Tab").with_id(|id| ids.push(id)))
+            .build();
+
+        assert!(!model.is_closable(ids[0]));
+
+        model.closable_set(ids[0], true);
+        assert!(model.is_closable(ids[0]));
+    }
+
+    #[test]
+    fn indent_operations() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("Item").with_id(|id| ids.push(id)))
+            .build();
+
+        assert_eq!(model.indent(ids[0]), None);
+
+        model.indent_set(ids[0], 2);
+        assert_eq!(model.indent(ids[0]), Some(2));
+
+        model.indent_remove(ids[0]);
+        assert_eq!(model.indent(ids[0]), None);
+    }
+
+    #[test]
+    fn divider_above_operations() {
+        let mut ids = Vec::new();
+        let mut model: Model<SingleSelect> = Model::builder()
+            .insert(|b| b.text("Item").with_id(|id| ids.push(id)))
+            .build();
+
+        assert_eq!(model.divider_above(ids[0]), None);
+
+        model.divider_above_set(ids[0], true);
+        assert_eq!(model.divider_above(ids[0]), Some(true));
+
+        model.divider_above_remove(ids[0]);
+        assert_eq!(model.divider_above(ids[0]), None);
+    }
+
+    #[test]
+    fn insert_returns_entity_mut() {
+        let mut model: Model<SingleSelect> = Model::builder().build();
+        assert_eq!(model.len(), 0);
+
+        let id = model.insert().text("Dynamic").id();
+        assert_eq!(model.len(), 1);
+        assert_eq!(model.text(id), Some("Dynamic"));
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Color picker model tests
+// ---------------------------------------------------------------------------
+mod color_picker_tests {
+    use cosmic::widget::color_picker::ColorPickerModel;
+    use iced_core::Color;
+
+    #[test]
+    fn color_picker_model_creation() {
+        // Verify model can be created without panicking
+        let _model = ColorPickerModel::new("Hex", "RGB", None, None);
+    }
+
+    #[test]
+    fn color_picker_with_initial_color() {
+        let color = Color::from_rgb(0.5, 0.3, 0.8);
+        let _model = ColorPickerModel::new("Hex", "RGB", Some(color), Some(color));
+    }
+
+    #[test]
+    fn color_picker_with_fallback() {
+        let fallback = Color::from_rgb(1.0, 0.0, 0.0);
+        let _model = ColorPickerModel::new("Hex", "RGB", Some(fallback), None);
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Theme tests
+// ---------------------------------------------------------------------------
+mod theme_tests {
+    use cosmic::theme;
+
+    #[test]
+    fn dark_theme_is_dark() {
+        let dark = &*theme::COSMIC_DARK;
+        assert!(dark.is_dark);
+    }
+
+    #[test]
+    fn light_theme_is_light() {
+        let light = &*theme::COSMIC_LIGHT;
+        assert!(!light.is_dark);
+    }
+
+    #[test]
+    fn high_contrast_dark_is_dark() {
+        let hc_dark = &*theme::COSMIC_HC_DARK;
+        assert!(hc_dark.is_dark);
+    }
+
+    #[test]
+    fn high_contrast_light_is_light() {
+        let hc_light = &*theme::COSMIC_HC_LIGHT;
+        assert!(!hc_light.is_dark);
+    }
+
+    #[test]
+    fn spacing_values_are_positive() {
+        let spacing = theme::spacing();
+        assert!(spacing.space_xxs > 0);
+        assert!(spacing.space_xs > 0);
+        assert!(spacing.space_s > 0);
+        assert!(spacing.space_m > 0);
+        assert!(spacing.space_l > 0);
+        assert!(spacing.space_xl > 0);
+        assert!(spacing.space_xxl > 0);
+    }
+
+    #[test]
+    fn default_theme_is_dark() {
+        assert!(theme::is_dark());
+    }
+
+    #[test]
+    fn active_theme_returns_valid_theme() {
+        let active = theme::active();
+        // Should not panic
+        let _cosmic = active.cosmic();
+    }
+}