Replicating the issue, Kubuntu 18/10 Xorg, Wayland not tested 1. map window switching to super+tab instead of alt+tab 2. map caps lock to super key with the following xmodmap setting: clear Lock keycode 66 = Super_L 3. Open two (preferably three as the issue happens more consistently) different application windows 4. Press down capslock, press and release tab key once, release capslock At this point the new window is active but kwin is still holding the "window selection" mode. It is as if kwin had not received the keyup event from the capslock key. Pressing any key makes the selector go away and the desired window to come up, but this swallows one keypress. If you instead press down capslock, then press down tab, then release capslock and then release tab, the switch works correctly. Originally tested on a PS/2 keyboard that does not have a super key using a PS/2-USB adapter but the issue is also present when using a regular USB keyboard. For comparison the same setup works without problems on Ubuntu 18/04 with the default GNOME desktop.
Please use the keyboard options to change the caps lock behavior to be a modifier instead of using xmodmap.
I can verify that that solution works. Thanks.