feat: csd_resize_borders hook, off by default

Every app got an 8px resize gutter on the inside of its surface, gated on
standard_csd() which defaults true. Under the cce compositor that duplicates
the server-side band just outside the window: two adjacent 8px gutters, two
cursor mechanisms, two code paths — and only the compositor's snaps to the
desktop grid. It also ate clicks, since a press inside the band starts a
grab and never reaches the widgets underneath, making anything flush to a
window edge unclickable in its outer 8px.

Split the resize band off standard_csd onto its own hook, defaulting off,
rather than flipping standard_csd itself: is_movable_backplate_at is checked
inside the same block, so turning standard_csd off would also have killed
drag-to-move for the six apps that override it (cce-ui, cce-fonts, cce-graph,
cce-colors, cce-files, cce-data-editor). standard_csd keeps gating the move
paths; only the edge grabs and their cursors move to the new hook.

With it off the press handler leaves the edge as None and falls through to
the titlebar and backplate move checks, so moving a window is unchanged.
cce-designer already overrides standard_csd to false and is unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

 src/backend/window_runner.rs | 36 ++++++++++++++++++++++++++++++------
 1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/src/backend/window_runner.rs b/src/backend/window_runner.rs
index 021b953..cab3275 100644
--- a/src/backend/window_runner.rs
+++ b/src/backend/window_runner.rs
@@ -1651,15 +1651,32 @@ pub trait Application: Sized + 'static {
     /// this for stateful relayout that can't wait for the next paint callback.
     fn handle_resize(&mut self, _width: f32, _height: f32, _scale: f64) {}
 
-    /// Whether the runner's built-in client-side decorations apply: the 8px
-    /// rect-edge resize grabs, the titlebar move band, and the matching edge
-    /// cursors. Return `false` for a window whose chrome doesn't follow its
-    /// rect (e.g. a circular pane) and drive moves/resizes yourself via
+    /// Whether the runner's built-in client-side decorations apply: the
+    /// titlebar move band, the movable-backplate drag regions, and — when
+    /// [`csd_resize_borders`](Application::csd_resize_borders) is also on —
+    /// the rect-edge resize grabs and their edge cursors. Return `false` for a
+    /// window whose chrome doesn't follow its rect (e.g. a circular pane) and
+    /// drive moves/resizes yourself via
     /// [`take_window_action`](Application::take_window_action).
     fn standard_csd(&self) -> bool {
         true
     }
 
+    /// Whether the standard CSD claims the outer 8px of the surface as resize
+    /// grabs (with matching edge cursors). Off by default: under the cce
+    /// compositor the server already provides a resize band just *outside* the
+    /// window, so enabling this gives a window two adjacent 8px gutters driven
+    /// by different code paths — and only the compositor's snaps to the
+    /// desktop grid. It also costs the app clicks, since a press inside the
+    /// band starts a grab and never reaches the widgets underneath.
+    ///
+    /// Turn it on for a window that must be resizable by its own edges under a
+    /// compositor that provides no such affordance. Only consulted when
+    /// [`standard_csd`](Application::standard_csd) is on.
+    fn csd_resize_borders(&self) -> bool {
+        false
+    }
+
     /// Whether the standard CSD reserves an implicit title-bar strip (`y` in `[8, 32)`) as a
     /// drag-to-move handle. Opt-in: off by default, so a window has no title bar and is moved
     /// through the compositor (or via explicitly-declared handles —
@@ -1811,7 +1828,10 @@ impl<A: Application> EngineState<A> {
         if let Some(icon) = inner.cursor_icon(lx, ly) {
             return icon;
         }
-        if inner.settings().app_id.starts_with("cce-status") || !inner.standard_csd() {
+        if inner.settings().app_id.starts_with("cce-status")
+            || !inner.standard_csd()
+            || !inner.csd_resize_borders()
+        {
             return CursorIcon::Default;
         }
         let border = 8.0f32;
@@ -2376,7 +2396,11 @@ impl<A: Application> PointerHandler for EngineState<A> {
                     if btn == MouseButton::Left && !is_status_bar && self.inner.as_ref().unwrap().standard_csd() {
                         let border = 8.0f32;
                         let mut edge = smithay_client_toolkit::reexports::protocols::xdg::shell::client::xdg_toplevel::ResizeEdge::None;
-                        if ly < border {
+                        if !self.inner.as_ref().unwrap().csd_resize_borders() {
+                            // Resize borders are off: the compositor's own band
+                            // outside the window handles it. Fall through to the
+                            // move checks so drag-to-move still works.
+                        } else if ly < border {
                             if lx < border {
                                 edge = smithay_client_toolkit::reexports::protocols::xdg::shell::client::xdg_toplevel::ResizeEdge::TopLeft;
                             } else if lx > self.logical_width - border {