gpui: Prevent multi-modifier gestures from dispatching standalone modifiers (#62036)

Resolves #61888.

Multi-modifier gestures could leave GPUI’s standalone-modifier state
armed after one modifier was released. For example, releasing `Alt`
before `Shift` during an `Alt+Shift` gesture caused the final `Shift`
release to be synthesized as a standalone shift keystroke.

This diff invalidates standalone-modifier synthesis whenever multiple
modifiers are active, while preserving genuine modifier-only key
bindings.

Release Notes:
- Fixed Alt+Shift gestures potentially triggering standalone Shift key
bindings.

crates/gpui/src/interactive.rs

@@ -835,7 +835,7 @@ mod test {
 
     use crate::{
         self as gpui, AppContext as _, Context, FocusHandle, InteractiveElement, IntoElement,
-        KeyBinding, Keystroke, ParentElement, Render, TestAppContext, Window, div,
+        KeyBinding, Keystroke, Modifiers, ParentElement, Render, TestAppContext, Window, div,
     };
 
     struct TestView {
@@ -902,4 +902,32 @@ mod test {
             })
             .unwrap();
     }
+
+    #[gpui::test]
+    fn test_multi_modifier_gesture_does_not_dispatch_standalone_modifier_binding(
+        cx: &mut TestAppContext,
+    ) {
+        let (test_view, cx) = cx.add_window_view(|_, cx| TestView {
+            saw_key_down: false,
+            saw_action: false,
+            focus_handle: cx.focus_handle(),
+        });
+
+        cx.update(|_, cx| {
+            cx.bind_keys(vec![KeyBinding::new("shift", TestAction, None)]);
+        });
+        test_view.update_in(cx, |test_view, window, cx| {
+            window.focus(&test_view.focus_handle, cx);
+        });
+
+        cx.simulate_modifiers_change(Modifiers::alt());
+        cx.simulate_modifiers_change(Modifiers::alt() | Modifiers::shift());
+        cx.simulate_modifiers_change(Modifiers::shift());
+        cx.simulate_modifiers_change(Modifiers::none());
+        assert!(!test_view.read_with(cx, |test_view, _| test_view.saw_action));
+
+        cx.simulate_modifiers_change(Modifiers::shift());
+        cx.simulate_modifiers_change(Modifiers::none());
+        assert!(test_view.read_with(cx, |test_view, _| test_view.saw_action));
+    }
 }

crates/gpui/src/window.rs

@@ -1149,7 +1149,7 @@ pub struct Window {
 #[derive(Clone, Debug, Default)]
 struct ModifierState {
     modifiers: Modifiers,
-    saw_keystroke: bool,
+    saw_other_input: bool,
 }
 
 /// Tracks input event timestamps to determine if input is arriving at a high rate.
@@ -5167,7 +5167,7 @@ impl Window {
         if let Some(event) = event.downcast_ref::<ModifiersChangedEvent>() {
             if event.modifiers.number_of_modifiers() == 0
                 && self.pending_modifier.modifiers.number_of_modifiers() == 1
-                && !self.pending_modifier.saw_keystroke
+                && !self.pending_modifier.saw_other_input
             {
                 let key = match self.pending_modifier.modifiers {
                     modifiers if modifiers.shift => Some("shift"),
@@ -5189,11 +5189,13 @@ impl Window {
             if self.pending_modifier.modifiers.number_of_modifiers() == 0
                 && event.modifiers.number_of_modifiers() == 1
             {
-                self.pending_modifier.saw_keystroke = false
+                self.pending_modifier.saw_other_input = false
+            } else if event.modifiers.number_of_modifiers() > 1 {
+                self.pending_modifier.saw_other_input = true
             }
             self.pending_modifier.modifiers = event.modifiers
         } else if let Some(key_down_event) = event.downcast_ref::<KeyDownEvent>() {
-            self.pending_modifier.saw_keystroke = true;
+            self.pending_modifier.saw_other_input = true;
             keystroke = Some(key_down_event.keystroke.clone());
             if key_down_event.keystroke.key_char.is_some()
                 && matches!(