I just installed Leap 42.1 (absolute Linux noob here, BTW), set it to use Plasma 5 (I think: I chose KDE during installation) and I’ve noticed one irritating behaviour: whenever there’s a window open, clicking on the K icon to invoke the Application Menu brings it up only for microseconds: too fast to do anything with it. Using the Alt-F1 key combo has the same effect: the menu won’t stay open. Is there a workaround for this?
Are you using a mouse or touchpad? I have had this and in my case it was an issue with the bluetooth mouse, fortunately my mouse is both bluetooth and usb so I swapped it to usb and it was fine. If you are using a touchpad might be worth trying a usb mouse if you have one.
I seem to have found the problem: I tinkered with the Bluetooth, as in disabled it entirely, as I don’t have any Bluetooth devices on this machine, and I don’t have a touchpad either (it’s a desktop system), so I disabled that too sometime last night. I also noticed last night that the problem seemed to have gone away somehow. Following your suggestion, I enabled the touchpad and Bluetooth services again in System Settings > Startup and Shutdown > Background Services. The problem came up immediately, BUT, even after disabling them services again, the swift disappearance of the Application Menu persisted.
The only way to make the Application Menu stick was to minimise all windows or to switch to a virtual desktop with no windows open on it. When I did this and clicked on an element on said Menu I noticed that the window wouldn’t come up to the fore but remain in the background; this pointed me to what I think the cause of the problem is: I had set my Focus Stealing Prevention to ‘High’ (System Settings > Windows Behaviour > Focus stealing prevention); I set it back to the default ‘Low’ and things are back to normal.
Thanks a lot! (And if this comes back I’ll repost.)