When clicking on edit in a form with a wysiwig widget inside, that widget would gain focus. It is not the expected behaviour (focusing the first field) and is really an issue on mobile, where the screen would scroll and the keyboard open.
The mouseup event was there to force summernote refreshing the editor toolbar (which will not display which buttons were active until the editor is focused). So removing it still has drawbacks (the toolbar is not up-to-date), but with chm it was decided to remove it as the issue with focus is worse.
Task-2047389 Closes#36360
MaxLarue
8ed94a11f4