How do I accept a modified version of Gemini's suggested changes

Gemini (on the right hand side of Firebase Studio) in Agent mode (is this IDX?) made a code suggestion.

When I do not click “Update File” but “Review Changes”, and make some changes, I cannot accept them because i find no button or menu item for that (see screenshot below).

If I click “Update File” in the Gemini chat after all, the version before my changes is taken.

If I instead do Ctrl-Shift-P “Accept All Incoming”, I get “no merge conflicts found in this file”.

So how do i accept my modified changes?