Add drop shadows and shrink the resize grab margin that was eating content clicks

Two independent daily-driving gaps closed in one pass, both from MISSING.md
and live user feedback:

Drop shadows (general.shadows, default true). Reuses the exact "bitmap
drawn outside geometry, cached like the border" technique border_strips/
render_border_top already established - decoration::shadow_bitmap rasterizes
a linear alpha falloff (Chebyshev/square-ring distance, not a true blur --
no blur primitive exists without a GPU shader, and the udev backend's
PixmanRenderer is software-only) from SHADOW_MAX_ALPHA (90/255, deliberately
subtle) at the window's own edge down to fully transparent SHADOW_SIZE (12px)
out. Cached in CompState::shadow_buffers, rebuilt at the same trigger points
as border_top_decorations (redraw_decoration_buffer), for the identical
damage-tracking reason: a fresh Id every frame means OutputDamageTracker
never finds a previous-frame match. No shadow for a maximized or fullscreen
window, matching the Hyprland/GNOME convention MISSING.md measures against.

Resize grab margin: 10px -> 6px (general.resize_margin, now configurable,
same call-site-count-preserving change as threading a new parameter through
one indirection point: ResizeEdge::hit_test's only production caller is
WindowManager::hit_test, so this didn't need touching every backend despite
hit_test being shared verbatim across X11/Wayland/Windows/macOS). Reported
live: ordinary clicks near any window edge - a link near a browser's edge,
a button near a panel's edge - regularly registered as a resize-edge grab
instead of reaching the client, not just an occasional near-miss, because
the 10px band was measured inward from the client's own content rect. 6px
stays comfortably grabbable while giving content back most of its edge.

Verified: cargo build --workspace (all 9 crates including the windows/macos
stub backends), cargo clippy --workspace (0 new warnings), cargo test across
core/wayland/config/x11 (188 tests, 0 failed). Shadow rendering verified at
the render-element level live in a nested session (correct geometry, alpha,
buffer contents) - grim/screencopy itself turned out to route through
winit.rs's separate capture_offscreen path, which only ever drew
`decorations` (titlebars), never borders or shadows, so screenshots taken
this way have never shown either; a real gap, not fixed in this pass.

1 files changed, 20 insertions, 0 deletions

diff --git a/crates/wayland/src/udev.rs b/crates/wayland/src/udev.rs
index 96d57d5..7e7a69f 100644
--- a/crates/wayland/src/udev.rs
+++ b/crates/wayland/src/udev.rs
@@ -312,6 +312,25 @@ impl CompState {
                     // windows) has to agree with what `sync_geometry` mapped
                     // the content to, or they drift apart again.
                     let geom = self.window_anims.get(&id).map(crate::state::WindowAnim::current_rect).unwrap_or(w.geometry);
+                    // Drawn first among this window's own decoration, and
+                    // positioned from the same animated `geom` as everything
+                    // else here - not `w.geometry` - for the identical
+                    // reason: a shadow that stayed at the pre-tween rect
+                    // while the window slid past it would look exactly as
+                    // detached as the border did before that fix. Not
+                    // fragment-clipped against `occluders` like the titlebar/
+                    // border below: at `SHADOW_MAX_ALPHA`'s low opacity, a
+                    // shadow bleeding slightly onto a window stacked in front
+                    // of this one reads as a soft edge, not the hard-line
+                    // bleed-through that made the titlebar/border need it.
+                    if let Some(shadow) = self.shadow_buffers.get(&id) {
+                        let rect = decoration::shadow_rect(geom);
+                        let pos = ((rect.x - origin.x) as f64, (rect.y - origin.y) as f64);
+                        match MemoryRenderBufferRenderElement::from_buffer(&mut udev.renderer, pos, shadow, None, None, None, Kind::Unspecified) {
+                            Ok(elem) => custom_elements.push(crate::elements::OverlayElement::Memory(elem)),
+                            Err(e) => log::warn!("udev: failed to import shadow buffer: {e}"),
+                        }
+                    }
                     if let Some(deco) = self.decorations.get(&id) {
                         // Fragment-clipped, same as the three solid border
                         // strips below - an *all-or-nothing* version of
@@ -996,6 +1015,7 @@ impl UdevPlatform {
             dead_layer_surfaces: HashSet::new(),
             decorations: HashMap::new(),
             border_top_decorations: HashMap::new(),
+            shadow_buffers: HashMap::new(),
             border_side_buffers: HashMap::new(),
             last_synced_size: HashMap::new(),
             pending: pending.clone(),