Upon further investigation, it seems to have something to do with either high resolutions or full-screen mode. Like if I set it to Windowed and 640x480 or something, then I can enable filters smoothly. However if I make the resolution go too high, ormake it full-screen again, it goes bonkers and shows super-zoomed in again. So it's not totally broken... probably someone's insight can make it work.
It also seems to be confused what resolution is what, in general. Like if I set it to Resizable, and then make the window cover almost the entire screen, the resolution it says it is using is not close to 1080p (whereas it should be close to 1920x1080)... it's more like 1200-by-something. That seems consistent with its behavior at 1920x1080p/full-screen: it shows a much larger image than 1920x1080.
EDIT: After messing with it more, Resizable with a filter on, and then manually dragged to be a bit smaller than the screen -- that's probably the best option at the moment for me. Everything is OK, except for the annoyances: difficult edge-scrolling, seeing the window title bar, taskbar appears if I touch bottom of screen... etc. Full-screen would be great.