gpui: Flash menu in menubar on macOS when action is triggered (#38588)

On macOS, traditionally when a keyboard shortcut is activated, the menu
in the menu bar flashes to indicate that the action was recognised.

<img width="289" height="172" alt="image"
src="https://github.com/user-attachments/assets/a03ecd2f-f159-4f82-b4fd-227f34393703"
/>

This PR adds this functionality to GPUI, where when a keybind is pressed
that triggers an action in the menu, the menu flashes.

Release Notes:

- N/A

crates/gpui/examples/set_menus.rs

@@ -1,13 +1,17 @@
 use gpui::{
-    App, Application, Context, Menu, MenuItem, SystemMenuType, Window, WindowOptions, actions, div,
-    prelude::*, rgb,
+    App, Application, Context, FocusHandle, KeyBinding, Menu, MenuItem, PromptLevel,
+    SystemMenuType, Window, WindowOptions, actions, div, prelude::*, rgb,
 };
 
-struct SetMenus;
+struct SetMenus {
+    focus_handle: FocusHandle,
+}
 
 impl Render for SetMenus {
     fn render(&mut self, _window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
         div()
+            .key_context("root")
+            .track_focus(&self.focus_handle)
             .flex()
             .bg(rgb(0x2e7d32))
             .size_full()
@@ -15,6 +19,16 @@ impl Render for SetMenus {
             .items_center()
             .text_xl()
             .text_color(rgb(0xffffff))
+            // Actions can also be registered within elements so they are only active when relevant
+            .on_action(|_: &Open, window, cx| {
+                let _ = window.prompt(PromptLevel::Info, "Open action fired", None, &["OK"], cx);
+            })
+            .on_action(|_: &Copy, window, cx| {
+                let _ = window.prompt(PromptLevel::Info, "Copy action fired", None, &["OK"], cx);
+            })
+            .on_action(|_: &Paste, window, cx| {
+                let _ = window.prompt(PromptLevel::Info, "Paste action fired", None, &["OK"], cx);
+            })
             .child("Set Menus Example")
     }
 }
@@ -23,24 +37,47 @@ fn main() {
     Application::new().run(|cx: &mut App| {
         // Bring the menu bar to the foreground (so you can see the menu bar)
         cx.activate(true);
+        // Bind keys to some menu actions
+        cx.bind_keys([
+            KeyBinding::new("secondary-o", Open, None),
+            KeyBinding::new("secondary-c", Copy, None),
+            KeyBinding::new("secondary-v", Paste, None),
+        ]);
         // Register the `quit` function so it can be referenced by the `MenuItem::action` in the menu bar
         cx.on_action(quit);
         // Add menu items
-        cx.set_menus(vec![Menu {
-            name: "set_menus".into(),
-            items: vec![
-                MenuItem::os_submenu("Services", SystemMenuType::Services),
-                MenuItem::separator(),
-                MenuItem::action("Quit", Quit),
-            ],
-        }]);
-        cx.open_window(WindowOptions::default(), |_, cx| cx.new(|_| SetMenus {}))
-            .unwrap();
+        cx.set_menus(vec![
+            Menu {
+                name: "set_menus".into(),
+                items: vec![
+                    MenuItem::os_submenu("Services", SystemMenuType::Services),
+                    MenuItem::separator(),
+                    MenuItem::action("Quit", Quit),
+                ],
+            },
+            Menu {
+                name: "File".into(),
+                items: vec![MenuItem::action("Open", Open)],
+            },
+            Menu {
+                name: "Edit".into(),
+                items: vec![
+                    MenuItem::action("Copy", Copy),
+                    MenuItem::action("Paste", Paste),
+                ],
+            },
+        ]);
+        cx.open_window(WindowOptions::default(), |_, cx| {
+            cx.new(|cx| SetMenus {
+                focus_handle: cx.focus_handle(),
+            })
+        })
+        .unwrap();
     });
 }
 
 // Associate actions using the `actions!` macro (or `Action` derive macro)
-actions!(set_menus, [Quit]);
+actions!(set_menus, [Quit, Open, Copy, Paste]);
 
 // Define the quit function that is registered with the App
 fn quit(_: &Quit, cx: &mut App) {