(Build ID: 20190614100022)

STR:

1. Go to twitter(https://twitter.com/FirefoxNightly e.g.)
2. Set zoom to 90%
3. click large image in tweet (to see the issue easily)

AR:

1. No close button
2. Partialy rendered(or missing) reply/retweet/favorite buttons/counts
3. Incorrect(much smaller) background shade of a tweet(not matched with client window), sometimes
4. and so on(?)(scrollbar vanished, sticked header short width, e.g. but I cannot say all these problems are concerned with this bug)

mozregression:
https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=a91efd693851d156f5680c88e6f91d12406ec3fe&tochange=94b369f38c536a4cc3b308311b1e88825e581ff5

I can also reproduce the issue on Nightly69.0a1 Windows10.
(Unfortunately, Bug 1559211 does not fix this.)

Ryan Hunt,
Can you please look into this?

Another STR

1. Open any Video page (incl. standalone/local video)
   ( e.g https://base-n.de/webm/VP9%20Sample.html )
2. Zoom out
3. Click on fullscreen icon at the end of video controls

Actual Results:
Right and bottom parts(incl. video controls) of video is truncated.

This change was suspect to not work under full page zoom, but I thought it
would be okay as it would only affect OOP-iframes. That was not true.

https://hg.mozilla.org/mozilla-central/rev/e593e11eff08

(In reply to Ryan Hunt [:rhunt] from comment #6)

Created attachment 9072532 [details]
Bug 1559518 - Revert composition size changes as they also affect top level
remote browsers. r?kats

This change was suspect to not work under full page zoom, but I thought it
would be okay as it would only affect OOP-iframes. That was not true.

(In reply to Pulsebot from comment #7)

Pushed by rhunt@eqrion.net:
https://hg.mozilla.org/integration/autoland/rev/e593e11eff08
Revert composition size changes as they also affect top level remote
browsers. r=kats

This did not fix anything on latest autoland build.
I can still reproduce the issue of comment#0, comment#4 and Bug 1559557.

App units of a remote browser element in the parent process are
different from app units inside the remote content in the child
process. We should apply the appropriate conversions by exposing
the relevant data as LayoutDevicePixel.

https://treeherder.mozilla.org/#/jobs?repo=try&author=rhunt%40eqrion.net

https://hg.mozilla.org/mozilla-central/rev/8d7491cb1c12

Issue confirmed with (Build ID: 20190614100022) on Windows 10.
Fix verified with 69.0b7 on Windows 10, macOS 10.13, Ubuntu 18.04.