+++ This bug was initially created as a clone of Bug #1656086 +++

STR:

1. Ensure that you're running in a browser with Fission enabled. (e.g. set fission.autostart to true in about:config and restart Firefox)

2. View this data URI:

data:text/html,<iframe src="https://www.w3.org/" style="width:500px">

EXPECTED RESULTS:
At the right side (next to the scrollbar), you should see a stripe with a blue gradient (which you'll also see if you directly visit https://www.w3.org/)

ACTUAL RESULTS::
There's a white stripe there instead.

Nightly-without-Fission and Chrome both give me EXPECTED RESULTS.
Nightly-with-Fission gives me ACTUAL RESULTS.

Regression range (from tnikkel in bug 1656086 comment 25, which I just double-checked with my STR here, & confirmed to still be correct):

https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=88db9bea4580df16dc444668f8c2cddbb3414318&tochange=bbbe694dbff25ce304833a9c2f6520ba9f61fb32

Simpler testcase (doesn't depend on w3c-hosted content):

data:text/html,<iframe src="https://bug1697890.bmoattachments.org/attachment.cgi?id=9208495">

EXPECTED RESULTS: Consistent lime border inside the iframe (with a little space set aside for a scroller at the right side)
ACTUAL RESULTS: lime border is missing on the right side; it's clipped. (It looks like we're reserving twice as much space as we actually need for the scrollbar, or something like that.)

Sometimes it looks like this. Is this the same bug?

I'm not sure; it might be. Can you reproduce that issue in an iframe in a data:text/html,<iframe src="...some yahoo URL here..."> testcase?

There may be still inconsistency uses of IsRootContentDocument().

I found the place.

https://hg.mozilla.org/mozilla-central/rev/82de87924bb9