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Yes, Dia has that "Arc feel", at least on macOS, it's very nice to use. Sadly, it still doesn't have all of Arc's features (I'll miss Boosts).

Is account mandatory? Do I remember correctly that Arc used to have and it was quite blocker.

Dia has a pretty hefty paid plan last time I checked, and so you will be using a browser constantly pushing you from free towards paid.

Yes. An Atlassian account is mandatory.