window-slot: Rework and document grab_focus() call

We'v been relying on a gtk_widget_grab_focus() call to avoid broken
focus states after view (re)loading.

But there is one particular case where we must not do it: if the
popover menu is shown. This is a rare situation which is currently
only possible to trigger by pressing [Spacebar] while the "Show
Hidden Files" menu item has focus.

But this currently relies on the slot calling a NautilusWindow method,
which is a layer violation which is getting in the way of reusing
NautilusWindowSlot in the FileChooser window[4].

Let's use generic GTK/GDK API to assess wether the menu is shown,
and drop the NautilusWindow method.

Originally I've just removed the grab_focus() call, but came to
realize it was still needed, so let's also document my findings here.

Part of: https://gitlab.gnome.org/GNOME/nautilus/-/work_items/3402

src/nautilus-window-slot.c

@@ -2502,6 +2502,24 @@ nautilus_window_slot_update_for_new_location (NautilusWindowSlot *self)
     nautilus_location_banner_load (self->banner, new_location);
 }
 
+static gboolean
+focus_is_on_popup (GtkRoot *window)
+{
+    GtkWidget *focus = gtk_root_get_focus (window);
+
+    if (focus != NULL)
+    {
+        GtkNative *native = gtk_widget_get_native (focus);
+
+        if (native != NULL)
+        {
+            return GDK_IS_POPUP (gtk_native_get_surface (native));
+        }
+    }
+
+    return FALSE;
+}
+
 static void
 view_started_loading (NautilusWindowSlot *self,
                       NautilusView       *view)
@@ -2511,12 +2529,29 @@ view_started_loading (NautilusWindowSlot *self,
         nautilus_window_slot_set_allow_stop (self, TRUE);
     }
 
-    /* Only grab focus if the menu isn't showing. Otherwise the menu disappears
-     * e.g. when the user toggles Show Hidden Files
-     */
-    if (!nautilus_window_is_menu_visible (self->window))
+    GtkRoot *window = gtk_widget_get_root (GTK_WIDGET (self));
+
+    if (!focus_is_on_popup (window))
     {
-        gtk_widget_grab_focus (GTK_WIDGET (self->window));
+        /* This mysterious gtk_widget_grab_focus() call has been carried over
+         * many refactorings, along the way having been wrapped in multiple ways
+         * (nautilus_view_grab_focus(), nautilus_window_pane_grab_focus(),
+         * nautilus_view_grab_focus()). It's been added in a series of bugfixes
+         * related to extra pane.[0] If I had to guess, I'd say it was to deal
+         * with side effects of the grandparent commit[1], which removed a much
+         * older bugfix[2].
+         *
+         * Regardless of its original purpose, we still rely on this call to
+         * prevent a "lost focus" situation. This is easily reproduced, in a
+         * build where this call is removed, by refreshing a view where nothing
+         * is selected. This "lost focus" state means, e.g., some keybindings,
+         * such as Ctrl+A, won't work.
+         *
+         * [0] Commit c69f3a2ba2d0bd23de5a218b8ce13d256481213a
+         * [1] Commit 4efd42312584b46f248e2839582a87776a7baebe
+         * [2] Commit 4f7ae827f7e13cf06965cc72ca60cf7801906a33
+         */
+        gtk_widget_grab_focus (GTK_WIDGET (window));
     }
 
     nautilus_window_slot_set_loading (self, TRUE);

src/nautilus-window.c

@@ -95,7 +95,6 @@ struct _NautilusWindow
 {
     AdwApplicationWindow parent_instance;
 
-    GtkWidget *app_button;
     GMenuModel *undo_redo_section;
 
     AdwTabView *tab_view;
@@ -2506,7 +2505,6 @@ nautilus_window_class_init (NautilusWindowClass *class)
 
     gtk_widget_class_set_template_from_resource (wclass,
                                                  "/org/gnome/nautilus/ui/nautilus-window.ui");
-    gtk_widget_class_bind_template_child (wclass, NautilusWindow, app_button);
     gtk_widget_class_bind_template_child (wclass, NautilusWindow, undo_redo_section);
     gtk_widget_class_bind_template_child (wclass, NautilusWindow, toolbar);
     gtk_widget_class_bind_template_child (wclass, NautilusWindow, split_view);
@@ -2632,16 +2630,3 @@ nautilus_window_search (NautilusWindow *window,
         g_warning ("Trying search on a slot but no active slot present");
     }
 }
-
-gboolean
-nautilus_window_is_menu_visible (NautilusWindow *self)
-{
-    GtkWidget *menu;
-
-    g_return_val_if_fail (NAUTILUS_IS_WINDOW (self), FALSE);
-
-    menu = GTK_WIDGET (gtk_menu_button_get_popover (GTK_MENU_BUTTON (self->app_button)));
-    g_return_val_if_fail (menu != NULL, FALSE);
-
-    return gtk_widget_is_visible (menu);
-}

src/nautilus-window.h

@@ -89,8 +89,6 @@ AdwTabView * nautilus_window_get_tab_view (NautilusWindow *window);
 
 void     nautilus_window_show_about_dialog    (NautilusWindow *window);
 
-gboolean    nautilus_window_is_menu_visible       (NautilusWindow *toolbar);
-
 /* sync window GUI with current slot. Used when changing slots,
  * and when updating the slot state.
  */

src/resources/ui/nautilus-window.ui

@@ -136,7 +136,7 @@
                       </object>
                     </property>
                     <child type="end">
-                      <object class="GtkMenuButton" id="app_button">
+                      <object class="GtkMenuButton">
                         <property name="tooltip-text" translatable="yes">Main Menu</property>
                         <property name="icon_name">open-menu-symbolic</property>
                         <property name="popover">