Anyone know what causes dark clouds to gather along the top edge of the
LCD?!

This calculator has been sitting on a shelf for years and when I tried to
use it found the batteries were flat, the seals had begun to sweat a little
and my first though was electrolyte contamination. Careful examination
showed the slight leakage was more or less confined to the individual button
cell compartments, the LCD internally has a Cd plated metal surround which
was pristine - no evidence of electrolyte contamination.

To make sure I dismantled the calculator and stuck all the buttons on double
sided tape to keep them in the right order while I scrubbed the casing with
detergent. The plastic window must be polarising as there was no sign of the
clouds with this off the LCD.

Another thing that makes me doubt electrolyte contamination is the cause,
the various enunciators along the top edge of the LCD seem to cut an overlap
out of the cloud effect. Ideas anyone?!

TIA.