In some circumstances, an absolutely positioned element with `display: flexbox` will cause a large paint (bottom right position of the element to the top left position of the viewport in Chrome, less in Safari, but still a larger area than the element itself). Attaching a testcase which is about as minimal as I could get it. To verify, open Web Inspector and use "Show paint rectangles" (or the Timeline), and hover the element that says "Hover". (The existence of opacity also seems to influence this, I don't see the same thing if I only change the background-color on hover for example.)
Created attachment 194528 [details] Demo of the bug
Safari, Chrome, and Firefox all agree on rendering for this test case. I don't believe there is any remaining compatibility issue.