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It falls apart how exactly? There's some app for which the popup menus are not shown at the right coordinates?

You are making this sound much more complicated than it really is.

Amongst many possible edge cases. Not to mention shearing with things that have rapid animations etc.

The trivial case is simple. You are imagining it's all the trivial case.

But they have already fixed shearing, otherwise there would be no MacBook display mirroring feature at all, multiple surfaces or not.

I’m talking about this open source project, not Apple’s implementation.