[teledyn]

the expansion must be microscopic because I was sitting with the unit held up between myself and the sun, with the open back door of a Dodge Caravan (not the glass window, but the door itself, steel and plastic) and the folliage of a large oak tree between us, and the ambient temperature outside was only perhaps 20C -- if I tilted the plane of the screen in any way within 90 degrees of the line to the sun, the screen would react, but if I stayed on the shadow side, everything was fine.

so I can't see heat-deformation as being a factor unless perhaps individual photons can cause localized deformations that will trigger the screen events.

hmmm ... so it is a real mystery then!