When I go full-screen in an application, buttons on the titlebar are vanished after returning to window mode.
For QT/KDE Applications (Kate with F11, smplayer) the close button disappears.
For Firefox (with F11) the collapse to titlebar button disappears.
I can reproduce this each time and I have this already since a couple of Plasma updates (can’t tell when it started). Can anyone reproduce it?
I’ve already reset my titlebar configuration (although it was default anyways), but to no avail.
I had some strange bugs maybe a year or so back with KDE.
My suggestion would be to make sure you have good backups and snapshots, then just do a fresh install and build it clean - importing only the configurations and files that you need, as and when you need them.
Actually, before you go that far, create a new user called TEST and see if that user has the same issues you do… If TEST doesn’t have the issue, then just delete everything in your /home folder except personal files (all KDE and .configs) and see how that goes - you can easily copy back from your Timeshift and Backup snapshots.
Are you using a custom theme? I cannot reproduce, I am using the standard Breeze window decoration, however I am on Manjaro unstable so I’m using:
KDE Plasma Version: 5.25.0
KDE Frameworks Version: 5.95.0
Qt Version: 5.15.4
Ok, I have to correct myself. I checked under another user - which doesn’t have the problem - and I found out I am not running the default titlebar configuration (pretty sure I checked this before…).
So when I set the default titlebar configuration I don’t have any issues. However once I add two buttons (see screenshot below) the problem (re-)appears.