Stop giving tiled windows a shadow that lands on their neighbour

Diagnosed by a peer session (dotfiles-1a): SHADOW_SIZE is 24px, and a
tiling layout with a small gap_inner (as little as 1px live) leaves the
shadow nowhere to fall except onto the adjacent tile, darkening it by up
to SHADOW_MAX_ALPHA (~35%) on whichever side is unfocused. Not a content
tint or an opacity rule - verified against the actual rasteriser and the
live rule set before accepting the diagnosis.

A drop shadow separates a window from what's behind it; tiled windows are
coplanar and adjacent by construction, with nothing behind them to
separate from. redraw_decoration_buffer's shadow gate now requires
w.floating in addition to the existing !maximized/!fullscreen checks.
DecorationSignature gained a floating field so toggling floating on its
own invalidates the decoration cache instead of waiting for an unrelated
field to force a rebuild.

Live-verified in a nested compositor: two tiled windows show a clean
shared edge with no gradient bleeding across; floating a window still
detaches it from the tile group with its shadow intact; shadows still
toggle globally both ways.

1 files changed, 9 insertions, 0 deletions

diff --git a/crates/wayland/src/state/mod.rs b/crates/wayland/src/state/mod.rs
index f39edcc..e07dc1d 100644
--- a/crates/wayland/src/state/mod.rs
+++ b/crates/wayland/src/state/mod.rs
@@ -114,6 +114,15 @@ pub(crate) struct DecorationSignature {
     pub(crate) maximized: bool,
     pub(crate) fullscreen: bool,
     pub(crate) shadows_enabled: bool,
+    /// A tiled window gets no shadow at all (see `redraw_decoration_
+    /// buffer`'s own gate) - included here for the same "one signature,
+    /// every real input" reason as `maximized`/`fullscreen` just above:
+    /// toggling floating on its own (`srd.window.toggle_floating()`,
+    /// switching layouts) changes nothing else this signature already
+    /// tracks, so without this the cache would keep serving whichever
+    /// shadow state happened to be cached until some unrelated field
+    /// (a resize, a focus change) forced a rebuild anyway.
+    pub(crate) floating: bool,
     /// Which of *this* window's own titlebar buttons (if any) is currently
     /// hovered, and the glyph-reveal animation's current progress (0..=255)
     /// - see `CompState::hovered_titlebar_button`'s own doc comment.