Show which account is being used with "single window for everything"
For me the "single window for everything" mode feels the most clean even if it sometimes confuses me when I have multiple accounts in the same MUC.
I think Gajim could add the JID to the end of "Write a message" in empty input bar or as I have suggested for other clients (Conversations & Dino), accounts could be given a small symbol which would then be shown in front of the tabs/contacts (which are very clear on Gajim though) to signify which account is being used.
I appear to be using Gajim 1.0.3 from flatpak.