Has anyone got an explanation, why only the outer frame of the Windows freeze? I guess for obvious gameplay reasons (you wouldn't see a thing with silent running and a few heat sinks), but still...Details are also some sensible explanations we can come up.
It's an interesting question. I'll make a few points and counterpoints (while waiting for coffee).
Firstly, the outer edge of the canopy is at the interface with the metallic frame/hull and therefore it could be assumed heat conduction is greater, therefore it will cool faster. The counterpoint to this is that there would normally be a flexible (normally insulating) material between canopy and frame/hull to allow flexibility during strain. The counter-counterpoint to that is that material technology in the 34th century would be far beyond 21st century thinking so all of the above may be thrown out the window (cough cough ahem).
Secondly, engineering would say that to avoid canopy damage, we would prefer the canopy to heat and cool evenly to prevent undue strain (uneven forces/pressures), therefore what we see at the outer edges is not at all ideal. The counterpoint to this is that engineers would have also calculated that the maximum possible strain/force/pressure on the canopy is well below the breaking point (tensile/flexile/compressive failure) and therefore uneven heating/cooling simply doesn't matter.
My best and favourite explanation though... it looks cool
Edit: As to why only the outer edge freezes, well it should gradually freeze towards the centre. How about an assumption that transparent electrical heating within the canopy keeps the middle FOV clear, or radiant heating from the HUD area, even ultrasonics perhaps - while life support is still enabled.