If you experienced problems like Launchpad Bug #1218735 where the cursor would be broken up into horizontal line artifacts and ghosting on Intel or Launchpad Bug #1218815 where there would be graphics glitches and screen corruption on Radeon with XMir, a workaround should now be in place. This is unrelated though to the Mir visual corruption when using Unity 8 on Mir without any XMir in the equation.
Revision 1063 of Mir was made on Thursday and it adds a new flag so that Mir buffers can attempt to figure out whether or not they are scanout-capable, i.e. could be rendered directly to the screen. While a buffer shouldn't need to know this, Mir is exposing that detail to clients in order to workaround the Radeon and Intel problems when using composite bypass support on XMir with regard to caching.
Beyond the Mir change, the xf86-video-intel driver package in Ubuntu was already modified so that it can make a better decision about cache usage based upon whether the buffer is to be scanned out to the screen. An xf86-video-ati change is also expected as part of this work.