Let auto-repeat kick in and you may well see the window being resized quickly but the shadow lagging behind, catching up (with many redraws) when you let go of the key.
Created attachment 641
Defer shadow recalculation until there are no pending configure events for the window which is being resized.
That patch hurts more than it helps, as the shadow disappears completely in many cases.
Revisions 22295 should fix the issue by preventing the X key events from flooding the event queue (which causes the visual effect you describe).