xdg: try to handle slow un-maximize with empty natural geometry better

In the case of an initially-maximized view which is taking a long time
to un-maximize (seen for example with Thunderbird on slow machines), we
may end up in handle_configure_timeout() with an empty pending geometry.
In that case we have no great options (we can't center the view since we
don't know the un-maximized size yet), so set a fallback position.

v2: check wlr_box_empty() before comparing pending and current

Fixes: #2191

include/view.h

@@ -13,6 +13,16 @@
 
 #define LAB_MIN_VIEW_HEIGHT 60
 
+/*
+ * Fallback view geometry used in some cases where a better position
+ * and/or size can't be determined. Try to avoid using these except as
+ * a last resort.
+ */
+#define VIEW_FALLBACK_X 100
+#define VIEW_FALLBACK_Y 100
+#define VIEW_FALLBACK_WIDTH  640
+#define VIEW_FALLBACK_HEIGHT 480
+
 /*
  * In labwc, a view is a container for surfaces which can be moved around by
  * the user. In practice this means XDG toplevel and XWayland windows.