What steps will reproduce the problem?
(1) Open a new tab
(2) Go to https://alexmos17.github.io/tests/fullscreen.html.  This contains a same-origin iframe with a div that be fullscreened.
(3) Click on the "Go fullscreen" button in the iframe.

What is the expected output?

The div should go fullscreen, with black background all around it.

What do you see instead?

The tab goes fullscreen with top-level frame still visible, the iframe turns black, and the div is not visible.  See screenshots.  It seems the fullscreen style was not applied to the <iframe> element.  

Note that if you hit escape to exit fullscreen and then click the "Go fullscreen" button again, the element will be correctly fullscreened on second try.

I reproed this on Mac Canary 51.0.2693.2 as well as on Linux ToT.

Bisecting this gets the following range:
https://chromium.googlesource.com/chromium/src/+log/8c912b533f434aa8509495bf0dd6e2dd177d0fce..9edddb9ded49023460a204675ee4e9bb763b9da7

Suspecting r382405.  rune@opera.com: can you please take a look?

 

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