This has always bothered me, I suspect it’s the same underlying reason most video players can’t do reverse frame-by-frame. But Quicktime allowed it twenty years ago, so it’s possible. I suppose you’d have to actually decode the entire keyframe interval and use the resulting frames as new “baked-in” keyframes so to say. I suppose that’s more or less what djv and other frame checkers do under the hood. But I don’t know what I’m talking about so…