Support tri-state (Mixed) for GTK checkbox menu items

Adds handling for the 'Mixed' state in checkbox menu items by using GTK's inconsistent state. The SetState and GetState methods now properly reflect and retrieve the tri-state visual and logical status.

Author: lijy91

src/platform/linux/menu_linux.cpp

@@ -344,13 +344,19 @@ void MenuItem::SetState(MenuItemState state) {
   pimpl_->state_ = state;
   if (pimpl_->gtk_menu_item_) {
     if (pimpl_->type_ == MenuItemType::Checkbox) {
+      // Update checked state
       gboolean active = (state == MenuItemState::Checked) ? TRUE : FALSE;
-      // Block the "toggled" signal to prevent recursive triggering
-      g_signal_handlers_block_by_func(G_OBJECT(pimpl_->gtk_menu_item_), 
+      // Block the "toggled" signal to prevent recursive triggering when setting active
+      g_signal_handlers_block_by_func(G_OBJECT(pimpl_->gtk_menu_item_),
                                       (gpointer)OnGtkCheckMenuItemToggled, this);
       gtk_check_menu_item_set_active(GTK_CHECK_MENU_ITEM(pimpl_->gtk_menu_item_), active);
-      g_signal_handlers_unblock_by_func(G_OBJECT(pimpl_->gtk_menu_item_), 
+      g_signal_handlers_unblock_by_func(G_OBJECT(pimpl_->gtk_menu_item_),
                                         (gpointer)OnGtkCheckMenuItemToggled, this);
+
+      // Reflect tri-state (Mixed) visually using GTK's inconsistent state
+      gtk_check_menu_item_set_inconsistent(
+          GTK_CHECK_MENU_ITEM(pimpl_->gtk_menu_item_),
+          (state == MenuItemState::Mixed) ? TRUE : FALSE);
     } else if (pimpl_->type_ == MenuItemType::Radio) {
       gboolean active = (state == MenuItemState::Checked) ? TRUE : FALSE;
       // Block the "toggled" signal to prevent recursive triggering
@@ -367,6 +373,15 @@ MenuItemState MenuItem::GetState() const {
   // For checkbox and radio items, get the actual state from GTK widget
   if (pimpl_->gtk_menu_item_ && 
       (pimpl_->type_ == MenuItemType::Checkbox || pimpl_->type_ == MenuItemType::Radio)) {
+    if (pimpl_->type_ == MenuItemType::Checkbox) {
+      // If inconsistent is set, treat as Mixed regardless of active
+      gboolean inconsistent = gtk_check_menu_item_get_inconsistent(
+          GTK_CHECK_MENU_ITEM(pimpl_->gtk_menu_item_));
+      if (inconsistent) {
+        return MenuItemState::Mixed;
+      }
+    }
+
     gboolean active = gtk_check_menu_item_get_active(GTK_CHECK_MENU_ITEM(pimpl_->gtk_menu_item_));
     return active ? MenuItemState::Checked : MenuItemState::Unchecked;
   }