One day after Canonical developers landed Mir composite bypass support
to dramatically improve the performance of full-screen games on Mir/XMir, Mir has picked up another important feature.
Daniel van Vugt has committed what he describes as "dramatically improved multi-monitor frame synchronization." For handling frame synchronization to avoid tearing on multi-monitor setups, a global frame count that's set to the highest refresh rate of all monitors is being used by Mir for determining synchronization. This new counter method tied to the highest refresh rate replaces their old sync logic of handling multi-monitor frame synchronization via timers.
This change went into Mir as of Revision 1040