GtkScaleButton: fix unwanted state propagation

GtkScaleButton/GtkVolumeButton use a popover
to show content. With the new GTK4 event model
clicking or hovering on that child popover produces
ACTIVE and PRELIGHT states that are propagated to
the main button, this is unwanted behaviour as the
button's appearance changes according to those.

We connect to the "state-flags-changed" signal of
GtkScaleButton, and remove the unwanted state flags
after verifying they're coming from the popover.

Fixes issue #2063

gtk/gtkscalebutton.c

@@ -157,6 +157,9 @@ static gboolean gtk_scale_button_scroll_controller_scroll (GtkEventControllerScr
                                                            gdouble                   dx,
                                                            gdouble                   dy,
                                                            GtkScaleButton           *button);
+static void     gtk_scale_button_state_flags_changed      (GtkWidget           *widget,
+                                                           GtkStateFlags        previous_state,
+                                                           gpointer             user_data);
 
 G_DEFINE_TYPE_WITH_CODE (GtkScaleButton, gtk_scale_button, GTK_TYPE_BUTTON,
                          G_ADD_PRIVATE (GtkScaleButton)
@@ -378,6 +381,8 @@ gtk_scale_button_init (GtkScaleButton *button)
 
   gtk_widget_init_template (GTK_WIDGET (button));
   gtk_popover_set_relative_to (GTK_POPOVER (priv->dock), GTK_WIDGET (button));
+  g_signal_connect (button, "state-flags-changed",
+                    G_CALLBACK (gtk_scale_button_state_flags_changed), priv->dock);
 
   /* Need a local reference to the adjustment */
   priv->adjustment = gtk_adjustment_new (0, 0, 100, 2, 20, 0);
@@ -498,6 +503,7 @@ gtk_scale_button_dispose (GObject *object)
   GtkScaleButton *button = GTK_SCALE_BUTTON (object);
   GtkScaleButtonPrivate *priv = gtk_scale_button_get_instance_private (button);
 
+  g_signal_handlers_disconnect_by_func (button, gtk_scale_button_state_flags_changed, priv->dock);
   g_clear_pointer (&priv->dock, gtk_widget_unparent);
 
   if (priv->click_id != 0)
@@ -1010,3 +1016,24 @@ gtk_scale_button_size_allocate (GtkWidget *widget,
 
   gtk_native_check_resize (GTK_NATIVE (priv->dock));
 }
+
+/* Removes ACTIVE and PRELIGHT states from the main button
+ * if they were propagated from the popover. Issue #2063 */
+static void
+gtk_scale_button_state_flags_changed (GtkWidget     *widget,
+                                      GtkStateFlags  previous_state,
+                                      gpointer       user_data)
+{
+  GtkWidget *popover = GTK_WIDGET (user_data);
+  GtkStateFlags state_popover = gtk_widget_get_state_flags (popover);
+  GtkStateFlags flags_to_unset = 0;
+
+  if (state_popover & GTK_STATE_FLAG_ACTIVE)
+    flags_to_unset |= GTK_STATE_FLAG_ACTIVE;
+
+  if (state_popover & GTK_STATE_FLAG_PRELIGHT)
+    flags_to_unset |= GTK_STATE_FLAG_PRELIGHT;
+
+  if (flags_to_unset)
+    gtk_widget_unset_state_flags (widget, flags_to_unset);
+}