Spinning this out from bug 1546362. This seems to be some kind of rounding issue where the video playing in the PiP player doesn't exactly fit the dimensions of the window when resizing it.
I think Chromium gets around this class of issue by making the underlying window fully transparent.
Reproduced the issue on latest Nightly 71 on Windows 10 x64.
Updating flags for affected versions.
This issue is still reproducible using Latest Nightly build 74.0a1 (02-04-2020), Beta 73.0 and Release version 72.0.2. Changed the flags accordingly.
I can reproduce this in latest nightly build , using arm64.