window: Improve transition between panels

Moving the headerbar to the panels accentuated a pre-existing
problem: transitioning between panels is flickery because we
immediately remove the old panel, while the new panel fades
in.

Improve this transition by delaying the removal of the old
panel to after the stack transition finishes.

2 files changed, 27 insertions, 5 deletions

diff --git a/shell/cc-window.c b/shell/cc-window.c
index 1411ee614..ac9afea94 100644
--- a/shell/cc-window.c
+++ b/shell/cc-window.c
@@ -62,6 +62,7 @@ struct _CcWindow
   AdwWindowTitle    *sidebar_title_widget;
   GtkStack          *stack;
 
+  GtkWidget  *old_panel;
   GtkWidget  *current_panel;
   char       *current_panel_id;
   GQueue     *previous_panels;
@@ -371,7 +372,6 @@ set_active_panel_from_id (CcWindow     *self,
   g_autofree gchar *name = NULL;
   CcPanelVisibility visibility;
   GtkTreeIter iter;
-  GtkWidget *old_panel;
   CcPanelListView view;
   gboolean activated;
   gboolean found;
@@ -397,7 +397,14 @@ set_active_panel_from_id (CcWindow     *self,
       CC_RETURN (TRUE);
     }
 
-  old_panel = self->current_panel;
+  if (self->old_panel)
+    gtk_stack_remove (self->stack, g_steal_pointer (&self->old_panel));
+
+  /* old_panel will be removed by the on_stack_transition_running_changed_cb
+   * callback - or, if panels changed before the transition ended, by the code
+   * just above.
+   */
+  self->old_panel = self->current_panel;
 
   gtk_tree_model_get (GTK_TREE_MODEL (self->store),
                       &iter,
@@ -428,9 +435,6 @@ set_active_panel_from_id (CcWindow     *self,
 
   CC_TRACE_MSG ("Current panel id: %s", start_id);
 
-  if (old_panel)
-    gtk_stack_remove (self->stack, old_panel);
-
   cc_panel_list_set_active_panel (self->panel_list, start_id);
 
   update_headerbar_buttons (self);
@@ -583,6 +587,22 @@ on_development_warning_dialog_responded_cb (CcWindow *self)
   gtk_widget_hide (GTK_WIDGET (self->development_warning_dialog));
 }
 
+static void
+on_stack_transition_running_changed_cb (GtkStack   *stack,
+                                        GParamSpec *pspec,
+                                        CcWindow   *self)
+{
+  gboolean transition_running;
+
+  CC_ENTRY;
+
+  transition_running = gtk_stack_get_transition_running (stack);
+
+  if (!transition_running && self->old_panel)
+    gtk_stack_remove (self->stack, g_steal_pointer (&self->old_panel));
+
+  CC_EXIT;
+}
 
 /* CcShell implementation */
 static gboolean
@@ -805,6 +825,7 @@ cc_window_class_init (CcWindowClass *klass)
 
   gtk_widget_class_bind_template_callback (widget_class, on_main_leaflet_folded_changed_cb);
   gtk_widget_class_bind_template_callback (widget_class, on_development_warning_dialog_responded_cb);
+  gtk_widget_class_bind_template_callback (widget_class, on_stack_transition_running_changed_cb);
   gtk_widget_class_bind_template_callback (widget_class, previous_button_clicked_cb);
   gtk_widget_class_bind_template_callback (widget_class, search_entry_activate_cb);
   gtk_widget_class_bind_template_callback (widget_class, show_panel_cb);
diff --git a/shell/cc-window.ui b/shell/cc-window.ui
index 6f0f3f979..07e1848e7 100644
--- a/shell/cc-window.ui
+++ b/shell/cc-window.ui
@@ -122,6 +122,7 @@
                 <property name="vexpand">True</property>
                 <property name="transition_type">crossfade</property>
                 <property name="width_request">360</property>
+                <signal name="notify::transition-running" handler="on_stack_transition_running_changed_cb" object="CcWindow" swapped="no" />
                 <style>
                   <class name="background"/>
                 </style>