vertical taskbar weirdness

[peyrol]

I use LMDE UP6 with kernel 3.7.0-4.dmz.1-liquorix-amd64 and I use mdm.
I put my taskbar on the right, and it is 40 pixels wide. When I have nine or fewer windows active in the same workspace, the system behaves itself, although I notice the icons on the taskbar that correspond to these windows are considerably smaller than they need to be. See the attached image. The icons are packed into less than half of the space available for them on my taskbar.
Once I exceed nine windows in the same workspace, the icons start flickering rapidly. If I kill some of the windows, the flickering might stop eventually. I can of course make it stop by rebooting. This problem does not occur when I place the taskbar on the bottom.
Is this a known bug? I would be very surprised if no one has mentioned this before.
Thanks in advance.