After the last update from QT from 4.8.5 to 4.8.6 i had some serious trouble with a wide range of applications: Some applications (like Clementine) made the desktop unresponsive, other just drew an empty window or had really large fonts applied.
A common error message was
QNativeImage: Unable to attach to shared memory segment.
often together with
X Error of failed request: BadDrawable (invalid Pixmap or Window parameter) Major opcode of failed request: 62 (X_CopyArea) Resource id in failed request: 0x0 Serial number of failed request: 598 Current serial number in output stream: 748
which made me look at the kernel's
$ sysctl -A|grep -P "shm(all|max)" kernel.shmall = 18446744073692774399 kernel.shmmax = 18446744073692774399
PAGE_SIZE of 4096. I'm pretty sure that i cannot afford to buy that mass of RAM.
I reduced both values to roughly half of size of my memory, but no effect.
After some hours of upgrading, downgrading, googling I came across the environment variable
QT_X11_NO_MITSHM=1 which instantly solved my problem. This variables disables the use of the MIT-SHM extension X11.
I haven't looked further to why this would by a problem, though.