SUMMARY I use a virtualbox for testing with KDE Neon. Initially it was 8 GB but that was a bit small, so I increased the virtual volume to 16 GB. Now I had to change the partition size to make use of the space. After starting the KDE partition manager, I increased the size to the maximum possible. After that I cannot decrease the size again. This might have been originally caused by something different.
Is filesystem ext4 and mounted? If yes then it's not a bug. ext4 does not support online shrinking. You need to unmount.
Changed status to waiting for info
Yes, it is ext4 and mounted. Thanks for the info, I did not know that ext4 can only be expanded, but not shrunk when mounted. Closing then as not a bug. (Still maybe as a feature request, some kind of hint for this special case where the "change size / move" window simply cannot do anything because the drive is already expanded to all available space might be nice.)
Hmm, the easiest place to add a small hint would be in file system support dialog. In other places, it's a bit trickier...
I had hoped that it could be included in the "change size / move" dialog when this happens, but can understand that it might be harder to implement. Just compared with gparted and they also just show nothing there when this happens.