I have had all the same issues you describe. Turning off hardware acceleration for the popout may help. See these posts:
https://forums.opera.com/post/109369
Posts made by taiichi-090
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Opera for WindowsRE: Video pop-out scaling problems and crash with YT clips
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Opera for WindowsRE: HTML5 video issues
You could try turning off hardware acceleration for the pop-out video, which seems to be in issue in the new version. (Actually, it's an issue for people's video hardware... but the distinction is academic here.)
Go to "opera://flags" and search for "Hardware accelerated detachable video" (or go right to "opera://flags/#media-detachable-view-hwaccelerated"). Switch it to Disabled if it's not. This may help for the time being.
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Opera for WindowsRE: Youtube Video pop out bug.
Go to "opera://flags", then search for "Hardware accelerated detachable video" and if the flag is Enabled, switch it to Disabled.
This seems to fix issues with the pop-out video viewer in the new version.