[REG 5.10 -> 5.11] Some content (e.g. embedded YouTube) is rendered wrong (tiled)

Some content seems to render in a scaled-down, tiled version with Qt 5.11 (reproducible with e.g. simplebrowser).

I can see it happen in this JIRA popup once all few seconds (where it goes back to normal a fraction of a second later) I attached a gif and a png of the frame where it happens.

A place where I can consistently reproduce the same kind of rendering (i.e. it stays that way and doesn't go back to normal) is clicking the YouTube link (leading to an unavailable video) on https://imgur.com/gallery/QgTr8 - I attached a screenshot as well.

However, I only seem to be able to reproduce this with my (developer) build of the current Qt 5.11 head - the beta 2 seems to be fine for some reason...