The reason I made those changes is because on 10.11 the OSX repaint behaviour seems to have changed, and the old code (where it told the window to display itself each time it resizes) began causing really nasty performance issues where the OS fails to batch-together a sequence of repaints, and instead queues and replays them all as you drag windows around.
So I changed it to do an asynchronous repaint when the window size changes. And it sounds like it's this async repaint that you're seeing, i.e. the OS is failing to repaint quickly enough and dropping a frame, which you're seeing.
Not really sure what to do about that. I can't put back the old code without bringing back the old bug..
Can you (or anyone) confirm whether this "glitch" happens on 10.11? If not then the answer is pretty easy: enable the old code on 10.10 and earlier, and the new version on 10.11. But I don't have 10.11 to test on right now..