Created attachment 370077 [details]
Elements larger than a flexbox container break out of the container and also break rendering of other elements when overflow is set to scroll. See the attached test case. The test case breaks in Safari, Safari TP and the latest Ephiphany preview. It also breaks in the latest Safari on iOS. I was unable to test with the latest Webkit Build Archive release since it seems to be non-functional on my macbook.
The text case renders as expected in the latest Chrome and Firefox browser on macOS and Windows.
Created attachment 370078 [details]
working render in chrome
Created attachment 370079 [details]
broken render in latest ephiphany technology preview