As of Plasma 5.10.x, there is now a one-pixel boundary right against the screen edge where task manager icons do not highlight and are not clickable. Unfortunately, these are the "biggest" pixels from the perspective of Fitt's Law--and the easiest to hit. Correct behavior was observed in 5.9.5, but I upgraded my main machine to 5.10.5 tonight (via Kubuntu 17.10) and noticed this regression. It seems to still be present in git master on KDE Neon.
Created attachment 108782 [details] Video illustrating the issue
Do you use a non-default theme? This is most likely a rounding issue causing the buttons to be 1px not tall enough. It's not generally true.
No, I use the standard Breeze theme. And the issue is reproducible at all widths, i.e. no matter how I resize the Icons-Only Task Manager panel, this still happens.
Looks like KWin's screen edge interfering. I had that in the top left corner due to a spurious touch screen edge being registered. Any chance of trying it on 5.11?
Yes, turning off the screen edge and rebooting fixes it on 5.10.5. Can't test on 5.11.x right now, sorry. Should we close this as FIXED or a duplicate of something already fixed?
Can't repro on 5.11.3.
*** This bug has been marked as a duplicate of bug 387775 ***