I suspect this is a blender bug. It clearly doesn't render at all to the
corrupted parts of its window, but AFAICT it doesn't use any of the
GLX_EXT_buffer_age, GLX_OML_swap_method or GLX_INTEL_swap_event extensions to
find out what if any guarantees there are about the contents of the back buffer
after a buffer swap. blender's assumptions about that may happen to be valid
with DRI2, which can only use up to two buffers, but not with DRI3, which can
use more.