SUMMARY
Turns out the width of the tag editor window and the whole gwenview depend on the maximal length of the existing tags that are autocompleted from the combo box pull down menu.
Thus when there is a tag with for example 400 chars (this is a real, valid case, produced for example in digikam with 20 levels subtags that are written as tag1/tag2/tag3/tagX etc by digikam), the editor window's width becomes huge and goes outside the screen. 
At the same time the whole gwenview window becomes distorted if in View->Information mode and goes beyond screen also. Additionally these widths cannot be manually controlled after they become huge.


STEPS TO REPRODUCE
1. Choose a picture and open it in gwenview
2. Go to View->Information mode
3. Open Tag editor and create and assign a tag with at least 400 chars -> leads to whole gwenview window width to go beyond screen and the information panel to be distorted
4. Close the tag editor and gwenview
5. Open gwenview and choose another picture that has not this tag assigned.
5. Open tag editor -> it is with huge and uncontrollable width

OBSERVED RESULT
1. Huge window widths of gwenview and the tag editor.
2. The widths of these windows cannot be made smaller manually

EXPECTED RESULT
1. Windows widths of the application and the tag editor to not depend on the length of the tags
2. The affected windows to be able to be manually made smaller

SOFTWARE/OS VERSIONS

Linux/KDE Plasma: 5.22.5
KDE Plasma Version: 5.22.5
KDE Frameworks Version: 5.86.0
Qt Version: 5.15.2
The xcb windowing system