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[kde] KDE Plasma taskbar frameborder not getting drawn correctly

Hello KDE-Community!
I've been plagued with a quite vexing problem, though being an estethic one.
After installing KDE Plasma 5 on my Linux Arch (which went pleasantly smooth), I've been tinkering with the settings and setting the taskbar-panel how I wanted it.
Some weeks after that, I noticed that the top frameborder of the taskbar stopped being drawn and the normally pleasing rounded corners went away as well.
Generally it seems that the upmost 2~3px of the taskbar is being clipped away...
When setting the taskbar to the horizontal edges of the screen, I can observe the same behaviour, although being less visual, but the top nevertheless.
To counteract this problem, I created another default panel; lo and behold! The panel got its borders again and the frameborder got drawn correctly.
The thing is: changing anything on the panel, even the slightest thing will totally mess up the frameborders, leaving it without any drawn border at times.
Even if I get the panel to draw its frameborder correctly, after logging out and in again will revert the painstakingly accomplished change...
I'm using the mesa graphics stack on an Intel i7 Sandybridge.
So, is there a way to fix this vexing problem?
If there is any data/logs you need, feel free to probe me.
Here are two screenshots of the taskbar/panel:
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