I think I got it.
-
Use fast edit but make it open the composer instead of the fast edit popup.
-
Then, select any text in the composer. The AI helper popup shows up.
-
Move the cursor to make the AI helper popup disappear, either by hovering this popup and move the cursor away or move the cursor away from the composer text area.
-
The selection is canceled.
If you close the composer, and edit the post without using the fast selection, the issue remains until you refresh the page.