I am running KDE Neon upgraded to the latest version (5.9.0) with updates being applied every day or so. The taskbar (ie: the windows list on the bottom on the screen which allows to you click & select a window) is running in grouped mode (not sure if this is relevant) and it regularly locks up. Sometimes it will go for a day or two without locking up, sometimes it takes an hour or so to lockup. When this happens, clicking on a window in the task bar has no effect (ie: the grouped window item is not expanded); sometimes it just takes a long time (like 10 seconds or so) for the window list to appear, sometimes nothing happens at all. My workaround for this right now is to kill plasmashell and the start a new instance (from command line) but obviously something is going wrong here. I don't know what additional info to give you; if you can provide instructions as to what additional info to gather, I will try and do so.
You chould check for high CPU usage using ksysguard/top, and then use a profiler like perf or a debugger like gdb to try and find out what it's busy with if confirmed.
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Dear Bug Submitter, This bug has been in NEEDSINFO status with no change for at least 30 days. The bug is now closed as RESOLVED > WORKSFORME due to lack of needed information. For more information about our bug triaging procedures please read the wiki located here: https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging Thank you for helping us make KDE software even better for everyone!