When you try to get the QR on a YouTube video in order to keep watching it on your mobile phone, you can't read said QR if you have the option "force dark pages" turned on in your Opera GX web browser. It's because the QR's border change from white to dark. A quickfix on the CSS should fix that. Meanwhile you have to keep the option "force dark pages" disabled or add YouTube URL to the list of exceptions in the "force dark pages" configuration for the QR to work.
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Opera GX bug on YouTube video PickUp feature (related to "force dark pages" beta feature)Opera GX