If you position absolutely off of the right-hand side of the document and then resize by dragging the left border on macOS, the div is very noticeably jittery. This is particularly noticeable in the menu on the right-hand side of the display in Dropbox Paper (https://paper.dropbox.com). I have a JSFiddle that exhibits the same behavior here: https://jsfiddle.net/24qvqd4a/. This doesn't look that good, and is noticeably worse in Firefox than in Chrome; it doesn't happen at all in Safari. In Firefox sometimes the items on the right-hand side of the page can appear to flicker, while in Safari the Dropbox paper website resizes very cleanly. In particular, if the window is resized quickly in Safari, everything resizes perfectly without jittering or flashing, while even at the slowest speed I could resize the window the blue box in the JSFiddle jitters around half an inch. Originally I suspected this was just due to Safari waiting until things were redrawn before resizing the actual window. Definitely the window itself resizes more choppily in Safari than in Chrome. But in Firefox the window also jitters a little while resizing. In any case, the window resizing choppily looks a lot nicer than objects oscillating back and forth (I wouldn't even notice it if I hadn't been looking carefully). I tried recording the bug, but it looks like the built-in screen recorder makes everything render infrequently enough that the jittering is no longer especially visible! So perhaps it will be even worse on 120fps displays.
status-firefox57: --- → wontfix
status-firefox58: --- → affected
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