Hi! I didn't want to switch away from Arc just to do my research, so I spent some time debugging and just wanted to drop a quick note that in Arc, you should go to
Arc -> Settings... -> Links -> Links from other apps open in Little Arc
Disable this setting, and I managed to complete the Google authentication with Arc.
(I'm not sure what problem you're referring to, but I don't think it's related to that other thread, so I've split this off to avoid further confusion.)
No, the original stated problem there was the Zotero Connector not being able to communicate with the desktop app via HTTP in both Chrome and Arc. That thread had nothing to do with Google (Docs?) authentication or whatever you're referring to here, and it had nothing to do with Arc specifically, since the person was having trouble in other browsers as well. Being the top Google result for some of the words in the thread does not make it related.
For this to be helpful, you'll need to take a step back and explain the actual issue that you were experiencing.
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Arc -> Settings... -> Links -> Links from other apps open in Little Arc
Disable this setting, and I managed to complete the Google authentication with Arc.
I wanted to add it there as it is the first thread that comes up when you Google Zotero Google authentication failing with Arc on macOS.
In case it still does not work, try to take the apps off full screen view.
For this to be helpful, you'll need to take a step back and explain the actual issue that you were experiencing.