Round the bottom border strip's corners to match the top

MISSING.md listed the border frame's bottom/left/right strips as
staying square while the top one (and the titlebar above it) rounds --
"unrelated to client content rounding... not attempted." The left/
right strips genuinely can't participate (border_strips' geometry has
them span only the height between the top and bottom strips, no
corner to round), but the bottom strip is exactly the same shape as
the top one and had no reason left to stay square.

decoration::render_border_bottom mirrors render_border_top exactly
(round_bottom_corners mirrors round_top_corners), cached the same way
in a new border_bottom_decorations map, and drawn in both render loops
via the same all-or-nothing occlusion check the top strip already
uses - pulled out of the left/right strips' per-fragment occlusion
splitting into its own dedicated bitmap path, matching top's existing
trade-off (cropping a rounded bitmap's source rect per fragment is
real extra work for a strip this thin) rather than inventing a new one.

One real bug caught before it shipped: round_bottom_corners' corner-
centre math (height - r - 1) panics on unsigned underflow whenever the
radius clamp lands on the strip's own full height (a real, common case
- a 2px-thick test strip hits it immediately). Fixed by computing the
centre as a signed offset instead, mirroring how the existing dx/dy
distance math already avoids the same class of issue.

1 files changed, 72 insertions, 3 deletions

diff --git a/crates/wayland/src/decoration.rs b/crates/wayland/src/decoration.rs
index 2888d69..42d3231 100644
--- a/crates/wayland/src/decoration.rs
+++ b/crates/wayland/src/decoration.rs
@@ -276,9 +276,13 @@ fn edge_distance(pos: u32, margin: u32, extent: u32) -> u32 {
 /// too, at a radius `width` pixels larger than the titlebar's (so the cut
 /// continues outward from the titlebar's own, rather than starting over),
 /// is what makes a bordered window's corner read as one continuous curve
-/// instead of a rounded titlebar sitting inside a square frame. The other
-/// three strips (bottom/left/right) don't participate in any visible
-/// corner and stay plain solid fills - see their render call sites.
+/// instead of a rounded titlebar sitting inside a square frame.
+///
+/// [`render_border_bottom`] gives the bottom strip the matching treatment
+/// for its own two corners. The left/right strips don't participate in any
+/// visible corner at all (`border_strips`' geometry has them span only the
+/// height *between* the top and bottom strips) and stay plain solid fills
+/// - see their render call sites.
 pub fn render_border_top(width: u32, thickness: u32, color: (u8, u8, u8)) -> Vec<u8> {
     let (width, thickness) = (width.max(1) as usize, thickness.max(1) as usize);
     let bg = rgb_to_bgra(color, 255);
@@ -290,6 +294,29 @@ pub fn render_border_top(width: u32, thickness: u32, color: (u8, u8, u8)) -> Vec
     buf
 }
 
+/// [`render_border_top`]'s mirror for the bottom strip - same construction,
+/// its own two corners (bottom-left/bottom-right) cut instead. Reported
+/// live, alongside the top-corner work: a bordered window's bottom two
+/// corners still read as square next to the now-rounded top ones, the same
+/// "inconsistently square" complaint that motivated rounding the top strip
+/// in the first place.
+///
+/// Handled as one all-or-nothing bitmap rather than folded into the
+/// left/right strips' per-fragment occlusion splitting (`visible_border_
+/// fragments`) - the same trade-off `render_border_top`'s own call site
+/// already makes and for the same reason: cropping a rounded bitmap's
+/// source rect per fragment is real extra work for a strip this thin.
+pub fn render_border_bottom(width: u32, thickness: u32, color: (u8, u8, u8)) -> Vec<u8> {
+    let (width, thickness) = (width.max(1) as usize, thickness.max(1) as usize);
+    let bg = rgb_to_bgra(color, 255);
+    let mut buf = vec![0u8; width * thickness * 4];
+    for px in buf.chunks_exact_mut(4) {
+        px.copy_from_slice(&bg);
+    }
+    round_bottom_corners(&mut buf, width, thickness, CORNER_RADIUS + thickness as u32);
+    buf
+}
+
 /// Renders a `width x height` BGRA8 buffer: filled with `background`, with
 /// `title` drawn left-aligned in `foreground` (best-effort glyph layout --
 /// no text shaping/kerning, adequate for the ASCII-heavy titles window
@@ -403,6 +430,37 @@ fn round_top_corners(buf: &mut [u8], width: usize, height: usize, radius: u32) {
     }
 }
 
+/// [`round_top_corners`]'s mirror for the bottom two corners - same
+/// construction, corner centres `r` *up* from the bottom instead of down
+/// from the top.
+fn round_bottom_corners(buf: &mut [u8], width: usize, height: usize, radius: u32) {
+    let r = (radius as usize).min(width / 2).min(height);
+    if r == 0 {
+        return;
+    }
+    // `cy` as a signed offset, not a `usize` - `height - r` can be exactly
+    // `0` (a strip whose radius clamp landed on its own full height, same
+    // as `round_top_corners` allows for `r == height`), which would
+    // underflow a plain `usize` subtraction one step further below.
+    let is_outside_corner = |x: usize, y: usize, cx: usize, cy: i64| -> bool {
+        let (dx, dy) = (x as i64 - cx as i64, y as i64 - cy);
+        (dx * dx + dy * dy) as u64 > (r * r) as u64
+    };
+    let cy = height as i64 - r as i64 - 1;
+    for y in (height - r)..height {
+        for x in 0..r {
+            if is_outside_corner(x, y, r, cy) {
+                buf[(y * width + x) * 4 + 3] = 0;
+            }
+        }
+        for x in (width - r)..width {
+            if is_outside_corner(x, y, width - r - 1, cy) {
+                buf[(y * width + x) * 4 + 3] = 0;
+            }
+        }
+    }
+}
+
 /// Sets one pixel to `color` if it falls inside the buffer - every icon
 /// drawn below goes through this so none of them need their own bounds
 /// checks.
@@ -687,6 +745,17 @@ mod tests {
     }
 
     #[test]
+    fn border_bottom_rounds_its_own_bottom_corners() {
+        let color = (0x40, 0x50, 0x60);
+        let (width, thickness) = (60, 2);
+        let buf = render_border_bottom(width, thickness, color);
+        let alpha_at = |x: usize, y: usize| buf[(y * width as usize + x) * 4 + 3];
+        assert_eq!(alpha_at(0, thickness as usize - 1), 0, "bottom-left corner pixel should be clipped");
+        assert_eq!(alpha_at(width as usize - 1, thickness as usize - 1), 0, "bottom-right corner pixel should be clipped");
+        assert_eq!(alpha_at(width as usize / 2, 0), 255, "centre of the strip must stay opaque");
+    }
+
+    #[test]
     fn context_menu_is_one_row_tall_per_item() {
         let items = [("Minimize", false), ("Maximize", false), ("Always on Top", false), ("Close", false)];
         let buf = render_context_menu(160, 28, &items, (0x2e, 0x34, 0x40), (0xff, 0xff, 0xff), (0x4c, 0x56, 0x6a), (0x10, 0x10, 0x10));