M3: give the kept button release back before the next press (Wayland)

The release a desktop grab kept from egui was appended to the next input,
so when that input carried the press of a second resize or move the
button ended the frame up and the drag stopped. It now goes before the
input's first primary press; only a release ahead of any press counts as
the kept one, since a quick click's release belongs to its own press.
Found by CodeRabbit in review of #3970.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017rPQFoRmQbw8vWhGgGVrRs

crates/ui-egui/src/window_controls.rs

@@ -247,15 +247,23 @@ pub fn desktop_grab(ctx: &egui::Context, command: egui::ViewportCommand) {
 }
 
 /// After `desktop_grab`, add the primary button's release to the next input, unless it brings one.
+/// The release goes before the input's first press (the next resize or move, or a click), which
+/// egui would otherwise end straight away; a release after that press is the press's own.
 pub fn release_after_desktop_grab(ctx: &egui::Context, raw: &mut egui::RawInput) {
     if ctx.data_mut(|d| d.remove_temp::<()>(grab_id())).is_none() {
         return;
     }
-    let released = raw.events.iter().any(|e| matches!(e, egui::Event::PointerButton { button: egui::PointerButton::Primary, pressed: false, .. }));
-    if !released {
-        let (pos, modifiers) = ctx.input(|i| (i.pointer.interact_pos().unwrap_or_default(), i.modifiers));
-        raw.events.push(egui::Event::PointerButton { pos, button: egui::PointerButton::Primary, pressed: false, modifiers });
-    }
+    let first = raw.events.iter().enumerate().find_map(|(n, e)| match e {
+        egui::Event::PointerButton { button: egui::PointerButton::Primary, pressed, .. } => Some((n, *pressed)),
+        _ => None,
+    });
+    let at = match first {
+        Some((_, false)) => return,
+        Some((n, true)) => n,
+        None => raw.events.len(),
+    };
+    let (pos, modifiers) = ctx.input(|i| (i.pointer.interact_pos().unwrap_or_default(), i.modifiers));
+    raw.events.insert(at, egui::Event::PointerButton { pos, button: egui::PointerButton::Primary, pressed: false, modifiers });
 }
 
 #[cfg(test)]
@@ -342,4 +350,43 @@ mod tests {
         super::release_after_desktop_grab(&ctx, &mut raw);
         assert_eq!(releases(&raw), 1, "not doubled when the input brings it");
     }
+
+    #[test]
+    fn the_release_goes_before_the_press_of_the_next_grab() {
+        let button = |pressed| egui::Event::PointerButton {
+            pos: Pos2::new(5.0, 5.0),
+            button: egui::PointerButton::Primary,
+            pressed,
+            modifiers: egui::Modifiers::NONE,
+        };
+        let primary = |raw: &egui::RawInput| {
+            raw.events
+                .iter()
+                .filter_map(|e| if let egui::Event::PointerButton { pressed, .. } = e { Some(*pressed) } else { None })
+                .collect::<Vec<_>>()
+        };
+        let ctx = egui::Context::default();
+        let frame = |events: Vec<egui::Event>, grab: Option<egui::ViewportCommand>| {
+            let mut raw = egui::RawInput { events, ..Default::default() };
+            super::release_after_desktop_grab(&ctx, &mut raw);
+            let order = primary(&raw);
+            ctx.run_ui(raw, |ui| {
+                if let Some(command) = grab.clone() {
+                    super::desktop_grab(ui.ctx(), command);
+                }
+            })
+            .textures_delta
+            .clear();
+            order
+        };
+        // The press of the first resize starts the grab; the desktop keeps its release.
+        frame(vec![egui::Event::PointerMoved(Pos2::new(5.0, 5.0)), button(true)], Some(egui::ViewportCommand::BeginResize(ResizeDirection::East)));
+        // The next input brings the press of the second resize.
+        assert_eq!(frame(vec![button(true)], None), [false, true], "the kept release, then the new press");
+        assert!(ctx.input(|i| i.pointer.primary_down()), "the second resize holds the button");
+
+        // A quick click's release belongs to its own press, not to the grab before it.
+        frame(vec![], Some(egui::ViewportCommand::StartDrag));
+        assert_eq!(frame(vec![button(true), button(false)], None), [false, true, false]);
+    }
 }