SUMMARY KWin offers the ability to move windows by holding the "alt" key. However, this does not stop alt from behaving as it normally would, which means as part of a shortcut to a menu item (e.g. "alt+f" to open the "file" menu). I have found this to happen especially frequently with Thunderbird. STEPS TO REPRODUCE 1. Press "alt" and drag a window 2. Stop pressing "alt" OBSERVED RESULT The dragged window behaves as if "alt" were still pressed. EXPECTED RESULT The dragged window behaves as if "alt" were not still pressed, as it is actually not. SOFTWARE/OS VERSIONS Linux: KDE neon KDE Plasma Version: 5.26.5 KDE Frameworks Version: 5.102.0 Qt Version: 5.15.8 ADDITIONAL INFORMATION
root cause same as in bug 444176 *** This bug has been marked as a duplicate of bug 444176 ***
Also, the modifier key for this is now Meta by default, not Alt. the Alt key suffered from various issues, including this one. So I would recommend just using the Meta key. If you didn't get that settings change automatically because you had previously customized it, you can set it manually at System Settings > Window Management > Window Actions tab.