Fix border corner rendered as a solid wedge instead of a ring

round_top_corners/round_bottom_corners only ever cut pixels *outside*
the shared corner radius (the rounded outer silhouette). Nothing cut
anything *inside* it, so a border strip's own "extra" rows (present
whenever corner_radius > border_width) stayed a solid filled disk out
to the centre column/row, then hit clip_middle_beyond_thickness's
hard, unblended rectangular cut right at the disk's own most opaque
point - a clean right-angle step, not a curve. Confirmed live,
zoomed: a real square notch bitten into an otherwise smooth arc,
reported as "squares on the inside corners of each vertex."

Added carve_inner_corner_pixel, the same smoothstep falloff as the
existing outer cut but inverted (cuts near the centre instead of far
from it), applied at radius - border_width so the corner becomes a
genuine ring of ~border_width visible thickness tapering smoothly to
transparent, instead of a filled wedge. Only render_border_top/
render_border_bottom pass an inner_radius; the titlebar's own corner
and the lock-screen box keep their existing solid-disk behaviour,
which is correct for a single flat-coloured panel with nothing of a
different colour underneath needing to show through.

Also generalizes round_top_corners with an explicit center_col
parameter, mirroring the existing center_row shift: the titlebar's own
corner circle was never shifted horizontally to match the border
strip's (only vertically), leaving a border_width-wide sliver of the
titlebar's own misaligned curve poking through at the seam.

border_top_and_titlebar_corners_meet_without_a_seam and
border_top_curve_actually_closes_within_the_side_strips_own_width
updated to match: both now compare the correct corresponding columns
(the titlebar's own buffer starts border_width columns inside the
border strip's), and the latter no longer demands exact 255 opacity at
a point that legitimately sits within the new inner cut's own
antialiasing band.

1 files changed, 12 insertions, 2 deletions

diff --git a/crates/wayland/src/decoration/border.rs b/crates/wayland/src/decoration/border.rs
index 472ef3b..9a2ed91 100644
--- a/crates/wayland/src/decoration/border.rs
+++ b/crates/wayland/src/decoration/border.rs
@@ -71,7 +71,14 @@ pub fn render_border_top(width: u32, thickness: u32, color: (u8, u8, u8), radius
     for px in buf.chunks_exact_mut(4) {
         px.copy_from_slice(&bg);
     }
-    round_top_corners(&mut buf, width, height, radius, radius as i32);
+    // `Some(radius - thickness)`: without this, the corner stayed a solid
+    // filled disk out to the centre column/row instead of a proper ring --
+    // see `corners::carve_inner_corner_pixel`'s own doc comment for the
+    // full story (reported live as "squares on the inside corners").
+    // `None` when `radius <= thickness` - no ring to carve, the corner is
+    // already exactly `thickness` px wide at most.
+    let inner_radius = (radius as usize > thickness).then(|| radius - thickness as u32);
+    round_top_corners(&mut buf, width, height, radius, radius as i32, radius as i32, inner_radius);
     clip_middle_beyond_thickness(&mut buf, width, radius as usize, thickness..height);
     buf
 }
@@ -127,7 +134,10 @@ pub fn render_border_bottom(width: u32, thickness: u32, color: (u8, u8, u8), rad
     // real corner. `render_border_top` gets `radius` used unshifted here
     // for the same reason it does: this buffer's own outermost row is
     // genuinely the true tip of the shape.
-    round_bottom_corners(&mut buf, width, height, radius);
+    // See `render_border_top`'s own matching comment for why this needs an
+    // inner radius too.
+    let inner_radius = (radius as usize > thickness).then(|| radius - thickness as u32);
+    round_bottom_corners(&mut buf, width, height, radius, inner_radius);
     // Extra rows sit above the original `thickness`, not below - the
     // bottom strip's curve resolves going *up* into content, the mirror of
     // the top strip's resolving *down* into it. See