Musescore 4.4 -- Header & footer popup list
The popup legend/list does not remain open when the cursor is placed in the selected space. The popup appears for a few seconds and then disappears, requiring user to move cursor away and into space again. Also, the popup doesn't appear unless the cursor is placed in just the right position.
Comments
Move your mouse off any/all of the header/footer fields, then move it back on to make it re-appear. It doesn't require any specific place when you move it back on, only that it be all the way off for a moment.
Sadly, yes, the list only appears for a few seconds then disappears. I believe that this is intended, but I sure wish they could find a better way to show this!!!!!
See https://github.com/musescore/MuseScore/issues/23241