Track horizontal scroll (which must be implemented by renderers)

src/buffer.rs

@@ -373,6 +373,29 @@ impl Buffer {
         }
 
         self.shape_until_scroll(font_system, prune);
+
+        // Adjust horizontal scroll to include cursor
+        if let Some(layout_cursor) = self.layout_cursor(font_system, cursor) {
+            if let Some(layout_lines) = self.line_layout(font_system, layout_cursor.line) {
+                if let Some(layout_line) = layout_lines.get(layout_cursor.layout) {
+                    if let Some(glyph) = layout_line.glyphs.get(layout_cursor.glyph) {
+                        //TODO: use code from cursor_glyph_opt?
+                        let x_a = glyph.x;
+                        let x_b = glyph.x + glyph.w;
+                        let x_min = x_a.min(x_b);
+                        let x_max = x_a.max(x_b);
+                        if x_min < self.scroll.horizontal {
+                            self.scroll.horizontal = x_min;
+                            self.redraw = true;
+                        }
+                        if x_max > self.scroll.horizontal + self.width {
+                            self.scroll.horizontal = x_max - self.width;
+                            self.redraw = true;
+                        }
+                    }
+                }
+            }
+        }
     }
 
     /// Shape lines until scroll

src/cursor.rs

@@ -132,7 +132,7 @@ pub enum Motion {
 }
 
 /// Scroll position in [`Buffer`]
-#[derive(Clone, Copy, Debug, Default, Eq, PartialEq, Ord, PartialOrd)]
+#[derive(Clone, Copy, Debug, Default, PartialEq, PartialOrd)]
 pub struct Scroll {
     /// Index of [`BufferLine`] in [`Buffer::lines`]. This will be adjusted as needed if layout is
     /// out of bounds
@@ -140,11 +140,17 @@ pub struct Scroll {
     /// Index of [`LayoutLine`] in [`BufferLine::layout`]. This will be adjusted as needed
     /// if it is negative or exceeds the number of layout lines
     pub layout: i32,
+    /// The horizontal position of scroll in fractional pixels
+    pub horizontal: f32,
 }
 
 impl Scroll {
-    /// Create a new cursor
-    pub const fn new(line: usize, layout: i32) -> Self {
-        Self { line, layout }
+    /// Create a new scroll
+    pub const fn new(line: usize, layout: i32, horizontal: f32) -> Self {
+        Self {
+            line,
+            layout,
+            horizontal,
+        }
     }
 }