Replied to your email with details - previously GM would resize and repaint the window every step of the way, which was a bad idea in some cases. Now GM waits until you’re done resizing and resizes the internal buffers after 10 frames, which is good for performance but looks weirder.
This can be addressed with an extension (by finding and modifying the internal countdown until resize and by hacking into the window’s background drawing respectively), but doing so requires an amount of research and experimentation.