This glitch started on Plasma 5.6 (and is still here in 5.6.1), I never experienced it before. On random occasions, the right side of the panel vanishes to show the desktop background. The size of the portion disappearing is not always the same (sometimes very short, sometimes longer). I don't know how to reproduce it as it seems random. I'll try to investigate to be able to reproduce it. Reproducible: Sometimes Arch linux, Plasma 5.6 and 5.6.1, Qt 5.6.0, linux-zen kernel 4.5.0, Intel Haswell integrated graphics with mesa 11.1.2.
I'm encountering the same problem. I'm using two monitors, each with panel containing task manager (the one with full names, not icon-only one) displaying only windows from particular monitor. The bug seems to be triggered by moving windows between monitors. Does not happen always, but usually is invoked within few window moves. Also disconnecting and reconnecting of external monitor sometimes triggers this bug.
Seems probability of occurrence is much higher in case the moved window is not the last one in tak manager, but the first one (in which case the relocation is causing shift of other task manager items).
I don't think the bug is related to a dual monitor configuration, since yesterday it happened on another computer with a single monitor and an nVidia card with proprietary drivers.
For me most often happens when starting or quitting an application (dolphin in particular)
Intel hd4000, 2 monitors (one conncted to dvi, other to vga) and after recent plasma update this bug start to happens very often to me: icons on task manager becomes invisible, keyboard layout indicator not showing layout changes e.t.c. and i cant get what triggers it. In system log i found this: file:///usr/lib/qt/qml/QtQuick/Controls/Button.qml:96: TypeError: Cannot read property of null file:///usr/share/plasma/plasmoids/org.kde.plasma.taskmanager/contents/ui/ToolTipDelegate.qml:220: TypeError: Cannot read property '0' of undefined
I can confirm this is happening on my setup as well (xf86-video-nouveau 1.0.12 with mesa 11.2.2).
I fixed this with complete kde reinstall (with wiping all configs and cache).
Happened just now by opening Firefox to reply here. If deleting cache and config files help, it would be good to figure out which file to delete, instead of everything in Plasma.
I have additional information I think. The bug occured using a custom Plasma theme (Antü). Since yesterday I switched back to Breeze Dark and the bug did not happen at all. It should be good to know if other users having this problem are using a custom Plasma theme too.
Someone with Qt 5.5 can reproduce? This is a Qt 5.6.0 bug, fixed in 5.6.1, if i'm not wrong.
Still present in Plasma 5.6.4, with Breeze default theme. The bug appear when an icon automatically leave the system tray, and disappear when the mouse overfly an window strip in the task manager. Reproductubility : always
I'm still see this behavior in Plasma 5.6.4 with the default Breeze theme.
The bug is fixed on my end.