This is NOT a "Light leak" Isue.
It Is an electronic Ground Shielding Isue
Most people would not even notice this
UNTILL you run CHDK with Long exposuresI have done alot to find the
Exact reason for this isue.
and i have found it.
if you use Black thick paper it will NOT fix the problem
i used tinfoil over the back of the CMOS about the size of the screen then poped the screen back in over it.
its a ground shield isue .
this is what mine WAS doing (Note the purple Haze) on the Left side.
i had the SAME isue to but i dont now:)