User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:69.0) Gecko/20100101 Firefox/69.0

Steps to reproduce:

Watched any video on tagesschau.de
like their livestream from ARD (German broadcaster)
https://www.tagesschau.de/multimedia/livestreams/livestream3/index.html

Actual results:

As you can see on the screenshot, the bottom part of the video (including the video control bar) is obscured by a blue rectangle.

I can still click/use the invisible video control bar if I know where each button is.

Pressing the windows key once shows the windows start menu and task bar but also makes the blue rectangle disappear.

Disabling hardware acceleration in Firefox settings doesn't change anything.

HW/SW info:
Intel i7 6700k
Nvidia GTX 980
Windows 10 Education Edition x64 1903 (up-to-date)
Nvidia Driver 418.91 (last driver with 3D support, will be supported till the end of the year, so up-to-date security-wise)

Expected results:

No blue bar like in Firefox release and Chrome release.

I just used Mozregression for a regressiontest:

2019-06-23T21:23:15: INFO : Narrowed inbound regression window from [6baa1883, 94b369f3] (3 builds) to [a91efd69, 94b369f3] (2 builds) (~1 steps left)
2019-06-23T21:23:15: DEBUG : Starting merge handling...
2019-06-23T21:23:15: DEBUG : Using url: https://hg.mozilla.org/integration/mozilla-inbound/json-pushes?changeset=94b369f38c536a4cc3b308311b1e88825e581ff5&full=1
2019-06-23T21:23:16: DEBUG : Found commit message:
Bug 1558482 - Restrict the composition size to the visibleRect for OOP-iframes. r=kats

Even if we don't have a root displayport, the composition size is still used for
displayport margins calculations. For extremely tall iframes, this will create
a displayport that is way to big. We should instead report a composition size that
is equivalent to the visible rect for OOP-iframes.

Differential Revision: https://phabricator.services.mozilla.com/D34528

2019-06-23T21:23:16: DEBUG : Did not find a branch, checking all integration branches
2019-06-23T21:23:16: INFO : The bisection is done.
2019-06-23T21:23:16: INFO : Stopped

The rectangles are actually part of the website (so they are only blue on this particular website)
screenshot2

my display is 2560x1440p, which might explain why this wasn't discovered earlier.
It definitely matters, which resolution the Firefox window has before I go into full screen mode

in maximized it shows the border as in Screenshot

in windowed mode at 2148x1276 it has rectangles on right and bottom of the video:
Screenshot3

1961x1276
Screenshot4

1961x1399
Screenshot5

the resolutions where chosen at random (you don't need to reproduce them accurately), I was able to measure the resolution with my screencapture tool

seems like the iframe size is calculated at a wrong point in time/needs to be recalculated when going into fullscreen mode.
https://bugzilla.mozilla.org/show_bug.cgi?id=1558482
But maybe there is an even more elegant solution :)

Closing this, as it's a duplicate of
https://bugzilla.mozilla.org/show_bug.cgi?id=1560272
(and https://bugzilla.mozilla.org/show_bug.cgi?id=1560082 )