Confirm Nemo's popup works; fix the two bugs that hid it, and shadow bleed across a monitor seam

Nemo's right-click context menu was the last open punch-list item, parked
twice as untestable. It works: verified end to end in a throwaway nested
compositor, menu and submenu both, at the correct position and stacking.
The popup path itself needed no fix, so the POPUP-GEOM-DIAG/POPUP-GRAB-DIAG
diagnostics are removed.

Two real bugs turned up in the way of testing it.

zwlr_virtual_pointer was a silent no-op on the winit backend. Every
Motion/MotionAbsolute handler read UdevState::bounds() behind an early
return when state.udev was None, and that field is Some only for the DRM
backend. The protocol advertised its global, accepted create_virtual_pointer
and accepted every request, then discarded all motion with no error and no
log. That is the backend a nested instance runs on, so the only safe way to
drive a throwaway compositor - a Wayland client of that compositor, which
cannot reach any other session, unlike ydotool's /dev/uinput writes - did
not work at all. Bounds now come from WindowManager::monitors() when udev is
absent; both backends fill that list from Platform::monitors().

The winit backend's screencopy pass rendered no popups and no shadows. It
re-renders the scene offscreen, and that second scene was missing tiers, so
grim on a nested instance reported the opposite of the truth: a menu drawing
perfectly on screen photographed as absent. The DRM backend never had this,
since it serves screencopy from the on-screen frame it just drew. Border
strips are still missing from that pass, called out in the code rather than
left silent.

Also fixed, from the "windows show a bit in the other monitor" report:
shadow_rect expanded by SHADOW_SIZE on every side with no monitor-boundary
awareness, so a window flush against a seam put its 24px shadow strip on the
neighbouring screen. shadow_rect_clipped clips to the bounding box of the
monitors the window's geometry actually touches - not just its assigned
one, since a window straddling a seam really does occupy both and clipping
there would cut its shadow off mid-body. The bitmap's own extent stays
unclipped, because the src rectangle indexes into it; only the fragment list
is clipped. Six tests on the incident's own numbers. Not confirmed on
screen: the nested backend cannot produce a second monitor.

New tool: tools/virtual-pointer-click, a scriptable virtual-pointer driver
that acknowledges each command after its round-trip, so a test script can
put a screenshot between a move and the click that follows it.

489 tests pass, clippy clean.

1 files changed, 7 insertions, 2 deletions

diff --git a/crates/wayland/src/winit/render.rs b/crates/wayland/src/winit/render.rs
index a07e074..dff425f 100644
--- a/crates/wayland/src/winit/render.rs
+++ b/crates/wayland/src/winit/render.rs
@@ -210,6 +210,10 @@ impl WaylandPlatform {
         // it correctly. The border strips and titlebar bitmap are
         // different: outside `geometry`, so they still need `occluders`'
         // explicit clip against whichever window is stacked in front.
+        // See the matching push in `udev/render.rs` - this backend only
+        // ever has the one output, so the clip is a no-op here, but reading
+        // the same source keeps the two shadow paths from drifting.
+        let monitor_bounds: Vec<srdwm_core::Rect> = self.wm.borrow().monitors().iter().map(|m| m.full_geometry).collect();
         let mut occluders: Vec<srdwm_core::Rect> = Vec::with_capacity(ids.len());
         for id in ids {
             let Some(w) = self.wm.borrow().window(id).cloned() else { continue };
@@ -405,11 +409,12 @@ impl WaylandPlatform {
             // exactly where two windows' corners nearly meet, which this
             // compositor's default cascade placement does constantly.
             if let Some(shadow) = self.state.shadow_buffers.get(&id) {
-                let rect = decoration::shadow_rect(frame);
+                let full = decoration::shadow_rect(frame);
+                let rect = decoration::shadow_rect_clipped(frame, &monitor_bounds);
                 for fragment in crate::elements::visible_border_fragments(rect, &occluders) {
                     let pos = (fragment.x as f64, fragment.y as f64);
                     let src = Rectangle::new(
-                        Point::from(((fragment.x - rect.x) as f64, (fragment.y - rect.y) as f64)),
+                        Point::from(((fragment.x - full.x) as f64, (fragment.y - full.y) as f64)),
                         Size::from((fragment.width as f64, fragment.height as f64)),
                     );
                     match MemoryRenderBufferRenderElement::from_buffer(renderer, pos, shadow, None, Some(src), None, Kind::Unspecified) {