Make corner resize reachable at the button corner, and widen it everywhere else

Reported live: "even where decorations are corner i should still be able
to corner resize, just its... hitbox... does not get in the way of the
close icon" - the titlebar corner holding Close/Maximize/Minimize had no
resize target at all, by design (competing with the close button was
judged worse than losing that one corner). The BUTTON_CLUSTER_MARGIN
strip between the button cluster and the frame's true edge was already
dead space no button claims, regardless of button_count - its own top
DECORATED_TOP_RESIZE_MARGIN rows now register as the diagonal corner
(TopLeft/TopRight) instead of falling through to Top/Drag, without
touching the button's own hitbox at all.

Also requested: widen the other three corners' own resize zone, since a
user reaching for a plain edge-resize instinctively aims for the middle
of that edge, not its corner - a bigger corner zone doesn't compete with
that instinct the way a bigger RESIZE_MARGIN would compete with ordinary
content clicks near an edge. CORNER_MARGIN raised from 3 to 5 (18px to
30px at the default resize_margin).

Updated one existing test whose own per-window resize_margin override
(30px) now put its plain-edge test point inside the widened corner zone
on a window too short for the two to stay apart - taller geometry, same
edge point relative to center, no change to what it actually verifies.
Added coverage for the new button-corner resize target on both sides,
and for the dead strip's own non-corner rows still just dragging as
before.

1 files changed, 11 insertions, 3 deletions

diff --git a/crates/core/src/manager/tests.rs b/crates/core/src/manager/tests.rs
index 9af0181..88699de 100644
--- a/crates/core/src/manager/tests.rs
+++ b/crates/core/src/manager/tests.rs
@@ -432,13 +432,21 @@
         wm.set_layout(wm.current_workspace(), "tiling"); // keeps add_window from overriding geometry via SmartPlacement
         let id = wm.alloc_window_id();
         let mut w = Window::new(id, "a");
-        w.geometry = Rect::new(100, 100, 400, 300);
+        // Tall enough that `CORNER_MARGIN * resize_margin`'s own widened
+        // corner zone (150px at this override, see `CORNER_MARGIN`'s own
+        // doc comment on why it's deliberately generous) doesn't reach
+        // anywhere near the plain-edge point tested below - a shorter
+        // window here used to work purely because the corner zone was
+        // narrower, not because this test cared about corners at all.
+        w.geometry = Rect::new(100, 100, 400, 800);
         w.resize_margin = Some(30);
         wm.add_window(w);
 
         // 15px in from the left edge: well past the WM-wide default (6px),
-        // but still inside this window's own wider 30px override.
-        let hit = wm.hit_test(115, 250);
+        // but still inside this window's own wider 30px override. Deep in
+        // the window's own vertical middle, well clear of either corner
+        // zone.
+        let hit = wm.hit_test(115, 500);
         assert_eq!(hit.map(|(_, h)| h), Some(TitlebarHit::Resize(ResizeEdge::Left)));
     }