In 2.1.50-mac-apple-qt6 and 2.1.51+rc2_a7cb5e21-mac-apple-qt6 everything opens in a new window (add, browser, edit …) and not in a tab. System preferences in Mac are set to prefer tabs. It’s working fine in anki 2.1.49 and other apps.
Thanks for your help!
The Qt 6 toolkit no longer supports the tab view, which has always been exclusive to Mac, by the way.
The Qt 5 build should still support tabs, but you can’t install that on an M1 system.
Unfortunately, there’s not much Anki can do about it other than wait for Qt to implement that again.
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Thanks for your feedback! No need to desperately search for a solution than 
(you can use the Qt 5 build on an M1 Mac, but it needs to run via Rosetta, so it will be slower)
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