ui: Add submenus to ContextMenu (#45743)

This PR introduces submenu functionality for Zed's dropdown/context
menus.

<img width="600" height="830" alt="Screenshot 2025-12-27 at 2  03@2x"
src="https://github.com/user-attachments/assets/eadfcd74-07fe-4a1f-be76-11b547c16dc8"
/>

```rs
.submenu("Trigger", |menu, _, _| {
    menu.entry("Item…", None, |_, _| {})
        .entry("Item…", None, |_, _| {})
})
```

Release Notes:

- N/A

---------

Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>

assets/keymaps/default-linux.json

@@ -55,6 +55,13 @@
       "down": "menu::SelectNext",
     },
   },
+  {
+    "context": "menu",
+    "bindings": {
+      "right": "menu::SelectChild",
+      "left": "menu::SelectParent",
+    },
+  },
   {
     "context": "Editor",
     "bindings": {

assets/keymaps/default-macos.json

@@ -54,6 +54,13 @@
       "ctrl-cmd-s": "workspace::ToggleWorktreeSecurity",
     },
   },
+  {
+    "context": "menu",
+    "bindings": {
+      "right": "menu::SelectChild",
+      "left": "menu::SelectParent",
+    },
+  },
   {
     "context": "Editor",
     "use_key_equivalents": true,

assets/keymaps/default-windows.json

@@ -54,6 +54,13 @@
       "down": "menu::SelectNext",
     },
   },
+  {
+    "context": "menu",
+    "bindings": {
+      "right": "menu::SelectChild",
+      "left": "menu::SelectParent",
+    },
+  },
   {
     "context": "Editor",
     "use_key_equivalents": true,

crates/menu/src/menu.rs

@@ -26,6 +26,10 @@ actions!(
         SelectFirst,
         /// Selects the last item in the menu.
         SelectLast,
+        /// Enters a submenu (navigates to child menu).
+        SelectChild,
+        /// Exits a submenu (navigates to parent menu).
+        SelectParent,
         /// Restarts the menu from the beginning.
         Restart,
         EndSlot,

crates/ui/src/components/dropdown_menu.rs

@@ -247,6 +247,30 @@ impl Component for DropdownMenu {
                 .entry("Option 4", None, |_, _| {})
         });
 
+        let menu_with_submenu = ContextMenu::build(window, cx, |this, _, _| {
+            this.entry("Toggle All Docks", None, |_, _| {})
+                .submenu("Editor Layout", |menu, _, _| {
+                    menu.entry("Split Up", None, |_, _| {})
+                        .entry("Split Down", None, |_, _| {})
+                        .separator()
+                        .entry("Split Side", None, |_, _| {})
+                })
+                .separator()
+                .entry("Project Panel", None, |_, _| {})
+                .entry("Outline Panel", None, |_, _| {})
+                .separator()
+                .submenu("Autofill", |menu, _, _| {
+                    menu.entry("Contact…", None, |_, _| {})
+                        .entry("Passwords…", None, |_, _| {})
+                })
+                .submenu_with_icon("Predict", IconName::ZedPredict, |menu, _, _| {
+                    menu.entry("Everywhere", None, |_, _| {})
+                        .entry("At Cursor", None, |_, _| {})
+                        .entry("Over Here", None, |_, _| {})
+                        .entry("Over There", None, |_, _| {})
+                })
+        });
+
         Some(
             v_flex()
                 .gap_6()
@@ -271,6 +295,14 @@ impl Component for DropdownMenu {
                             ),
                         ],
                     ),
+                    example_group_with_title(
+                        "Submenus",
+                        vec![single_example(
+                            "With Submenus",
+                            DropdownMenu::new("submenu", "Submenu", menu_with_submenu)
+                                .into_any_element(),
+                        )],
+                    ),
                     example_group_with_title(
                         "Styles",
                         vec![