shell: Fix artifacts caused by workspace change animation

Views that extend past the bottom of the output are still visible after
the workspace animation ends but before its layer is hidden. When the
layer was hidden, nothing would cause those regions to be repainted,
leading to artifacts.

https://bugs.freedesktop.org/show_bug.cgi?id=78363

desktop-shell/shell.c

@@ -1027,8 +1027,17 @@ finish_workspace_change_animation(struct desktop_shell *shell,
 				  struct workspace *from,
 				  struct workspace *to)
 {
+	struct weston_view *view;
+
 	weston_compositor_schedule_repaint(shell->compositor);
 
+	/* Views that extend past the bottom of the output are still
+	 * visible after the workspace animation ends but before its layer
+	 * is hidden. In that case, we need to damage below those views so
+	 * that the screen is properly repainted. */
+	wl_list_for_each(view, &from->layer.view_list, layer_link)
+		weston_view_damage_below(view);
+
 	wl_list_remove(&shell->workspaces.animation.link);
 	workspace_deactivate_transforms(from);
 	workspace_deactivate_transforms(to);