The problem is caused by commit d19641fc01f77420b4ec5d6db6723f519f8f8272 which fixes bug #1602561.
That commit deletes the code that removes the XRandR outputs associated with the XMir screen after a resize. That means that whatever XRandR configurations are created at XMir startup remain in place for the lifetime of the process even if they do not correspond to the actual size after an X11 resize and rotation (XRandR) and input events get transformed through a matrix corresponding to the original size of the screen. When all the outputs were deleted previously, the identity matrix is assumed for input remapping.
The problem is caused by commit d19641fc01f7742 0b4ec5d6db6723f 519f8f8272 which fixes bug #1602561.
That commit deletes the code that removes the XRandR outputs associated with the XMir screen after a resize. That means that whatever XRandR configurations are created at XMir startup remain in place for the lifetime of the process even if they do not correspond to the actual size after an X11 resize and rotation (XRandR) and input events get transformed through a matrix corresponding to the original size of the screen. When all the outputs were deleted previously, the identity matrix is assumed for input remapping.