Created attachment 131825 [details]
screenshot showing pre-filled text

SUMMARY
This issue started appearing about a week ago in Neon unstable builds: when opening the tag editor, the text edit box at the bottom is pre-filled with some text, so that if you want to put in another tag you have to first delete what is in there. The pre-filled text seems to be the first tag, in alphabetical order, from the Baloo tag database. As I have several hundred different tags in use on my system, the pre-populated text is very unlikely to be useful and it just adds another hassle.

STEPS TO REPRODUCE
1. Have some tags already in the system and indexed.
2. Open an image file with Gwenview
3. Click "Edit" in the info pane under "Tags" or hit CTRL+T

OBSERVED RESULT
The tag editor's text box already has some text in it, which is unselected and the cursor positioned immediately after that - so any editing that isn't accepting the pre-filled text has to start by erasing the text.

EXPECTED RESULT
The tag editor's text box should be focused and empty.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
KDE Plasma Version:  5.19.90
KDE Frameworks Version: 5.75.0
Qt Version: 5.15.0

ADDITIONAL INFORMATION
I'm not sure what was the intent with having some text pre-filled, but if it is to speed up entry (and I think that is misguided as the chosen pre-filled text is very unlikely to be usable) then having that text also selected would be a better behavior - as quick entry can still be accomplished by just hitting ENTER while if the user is not interested in the pre-filled text, they can just start typing.

I still think that choosing an arbitrary tag to be pre-filled is more confusing than helpful and the suggestion drop down menu is enough of a help for quick entry.