element/wayland: Add blur background-effect

src/wayland/handlers/background_effect.rs

@@ -0,0 +1,59 @@
+use smithay::{
+    reexports::wayland_server::{DisplayHandle, protocol::wl_surface::WlSurface},
+    utils::{Logical, Rectangle},
+    wayland::{
+        background_effect::{Capability, ExtBackgroundEffectHandler},
+        compositor::{Cacheable, RectangleKind, RegionAttributes, with_states},
+    },
+};
+
+use crate::state::State;
+
+#[derive(Debug, Clone, Default)]
+pub struct ComputedBlurRegionCachedState {
+    /// Region of the surface that will have its background blurred.
+    pub blur_region: Option<Vec<Rectangle<i32, Logical>>>,
+}
+
+impl Cacheable for ComputedBlurRegionCachedState {
+    fn commit(&mut self, _dh: &DisplayHandle) -> Self {
+        self.clone()
+    }
+
+    fn merge_into(self, into: &mut Self, _dh: &DisplayHandle) {
+        *into = self;
+    }
+}
+
+impl ExtBackgroundEffectHandler for State {
+    fn capabilities(&self) -> Capability {
+        Capability::Blur
+    }
+
+    fn set_blur_region(&mut self, surface: WlSurface, region: RegionAttributes) {
+        with_states(&surface, |states| {
+            let mut blur_state = states.cached_state.get::<ComputedBlurRegionCachedState>();
+
+            blur_state.pending().blur_region = Some({
+                let (added, subtracted) = region
+                    .rects
+                    .iter()
+                    .cloned()
+                    .partition::<Vec<_>, _>(|(op, _)| matches!(op, RectangleKind::Add));
+                let added = added.into_iter().map(|(_, rect)| rect).collect::<Vec<_>>();
+                Rectangle::subtract_rects_many_in_place(
+                    added,
+                    subtracted.into_iter().map(|(_, rect)| rect),
+                )
+            })
+        })
+    }
+
+    fn unset_blur_region(&mut self, surface: WlSurface) {
+        with_states(&surface, |states| {
+            let mut blur_state = states.cached_state.get::<ComputedBlurRegionCachedState>();
+
+            blur_state.pending().blur_region.take();
+        })
+    }
+}

src/wayland/handlers/mod.rs

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 pub mod a11y;
+pub mod background_effect;
 pub mod buffer;
 pub mod compositor;
 pub mod corner_radius;