Mirror layers can have a different device scale factor from the original layer. However the layer delegates remain the same. This causes issues with the PaintCache that views::View (A layer delegate) uses during paint. 

Cache entry done by the mirrored layer may be used by the original layer during paint(or vice versa). If the two layers are at different device scale factor, this can lead to an invalid cache entry being used.

A direct consequence of this is seen in the phantom window bug 786628