Manual Regression Range:
Good Build: 51.0.2686.0
Bad Build: 51.0.2687.0

Providing Change log url as  'WidevineCdm component' is not available in chromium builds:
https://chromium.googlesource.com/chromium/src/+log/51.0.2686.0..51.0.2687.0?pretty=fuller&n=10000 

Suspecting: r382405 ?

Please re-assign if your change is not the cause of this issue. 

Deleted: Actual.mp4 1.2 MB

Actual.mp4 1.2 MB Download

Deleted: Expected.mp4 557 KB

Expected.mp4 557 KB Download

That's most certainly r382405. I'll probably have to revert to using a full style recalc, but I'd like to keep the change where we don't check for loaded UA style for every element, so please don't revert. I'll upload a new CL.

Adding release block label, please undo if not the case.

So, I cannot reproduce this as I cannot build Chrome, thus I don't have WidevineCdm. Have you seen problems with other fullscreen'ed video? I have tested youtube which didn't seem to have a problem.

Patch for review: https://codereview.chromium.org/1823143002/

Still able to reproduce the issue on Windows 7, Mac 10.10.5, Ubuntu 14.04 using latest M-51(51.0.2692.0).Observed that Video full screen is not proper.
Please find attached screencast.

rune@Could you please look into this.

Deleted: 596803.mp4 1.2 MB

596803.mp4 1.2 MB Download

 Issue 598276  has been merged into this issue.

https://codereview.chromium.org/1823143002/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7a71fcc137ef2e4a78c93ecb15e1623ca9b3eb65

commit 7a71fcc137ef2e4a78c93ecb15e1623ca9b3eb65
Author: rune <rune@opera.com>
Date: Tue Mar 29 13:41:43 2016

Ensure fullscreen.css loaded for ancestor invalidation

Using invalidation sets caused regression  crbug.com/596803  because we
only ensured the fullscreen.css had features available for style
resolving in the fullscreened element's document. This CL ensures the
features are up-to-date for all fullscreen related pseudoStateChanged.

I was not able to reproduce the problem in 596803, but 448721 also
regressed and I've confirmed this CL fixes that regression.

The added layout test does not fail without this fix because the
full screen implementation in content_shell is different and
setMediaType() for fullscreen on resize causes a full recalc of
everything in content_shell before we try to apply fullscreen style
changes. However, if mediaQueryAffectingValueChanged was smarter when
changing media type to fullscreen. That test would have failed.

BUG= 596803 

Review URL: https://codereview.chromium.org/1823143002

Cr-Commit-Position: refs/heads/master@{#383711}

[add] https://crrev.com/7a71fcc137ef2e4a78c93ecb15e1623ca9b3eb65/third_party/WebKit/LayoutTests/fullscreen/full-screen-iframe-ua-style-expected.txt
[add] https://crrev.com/7a71fcc137ef2e4a78c93ecb15e1623ca9b3eb65/third_party/WebKit/LayoutTests/fullscreen/full-screen-iframe-ua-style.html
[modify] https://crrev.com/7a71fcc137ef2e4a78c93ecb15e1623ca9b3eb65/third_party/WebKit/Source/core/dom/Element.cpp
[modify] https://crrev.com/7a71fcc137ef2e4a78c93ecb15e1623ca9b3eb65/third_party/WebKit/Source/core/dom/Fullscreen.cpp

 Issue 598911  has been merged into this issue.

 Issue 598187  has been merged into this issue.