Switch: Implement :active state on slider subnode

https://gitlab.gnome.org/GNOME/gtk/issues/1201

gtk/gtkswitch.c

@@ -195,6 +195,21 @@ gtk_switch_begin_toggle_animation (GtkSwitch *sw)
     }
 }
 
+static void
+gtk_switch_set_slider_state_active (GtkSwitch *sw,
+                                    gboolean   active)
+{
+  GtkSwitchPrivate *priv = sw->priv;
+  GtkStateFlags state = gtk_widget_get_state_flags (GTK_WIDGET (sw));
+
+  if (active)
+    state |= GTK_STATE_FLAG_ACTIVE;
+  else
+    state &= ~GTK_STATE_FLAG_ACTIVE;
+
+  gtk_css_gadget_set_state (priv->slider_gadget, state);
+}
+
 static void
 gtk_switch_multipress_gesture_pressed (GtkGestureMultiPress *gesture,
                                        gint                  n_press,
@@ -214,6 +229,8 @@ gtk_switch_multipress_gesture_pressed (GtkGestureMultiPress *gesture,
   if ((priv->is_active && x <= allocation.width / 2.0) ||
       (!priv->is_active && x > allocation.width / 2.0))
     gtk_gesture_set_state (priv->pan_gesture, GTK_EVENT_SEQUENCE_DENIED);
+  else
+    gtk_switch_set_slider_state_active (sw, TRUE);
 }
 
 static void
@@ -226,6 +243,7 @@ gtk_switch_multipress_gesture_released (GtkGestureMultiPress *gesture,
   GtkSwitchPrivate *priv = sw->priv;
   GdkEventSequence *sequence;
 
+  gtk_switch_set_slider_state_active (sw, FALSE);
   sequence = gtk_gesture_single_get_current_sequence (GTK_GESTURE_SINGLE (gesture));
 
   if (priv->in_switch &&
@@ -258,6 +276,7 @@ gtk_switch_pan_gesture_pan (GtkGesturePan   *gesture,
   priv->handle_pos = CLAMP (offset, 0, 1.0);
 
   /* we need to redraw the handle */
+  gtk_switch_set_slider_state_active (sw, TRUE);
   gtk_widget_queue_allocate (widget);
 }
 
@@ -289,6 +308,7 @@ gtk_switch_pan_gesture_drag_end (GtkGestureDrag *gesture,
     return;
 
   priv->handle_pos = active ? 1.0 : 0.0;
+  gtk_switch_set_slider_state_active (sw, FALSE);
   gtk_switch_set_active (sw, active);
   gtk_widget_queue_allocate (GTK_WIDGET (sw));
 }