Unexpected rectangles appearing in background

[radiovisual]

Ever since the upgrade to Electron version 1.2.2, there are randomly-sized rectangles appearing in the background of the ruler windows (notice the darker region in the lower left of the ruler window):

[radiovisual]

They appear during resize, and then vanish after resize.

[radiovisual]

There is no apparent pattern to when they appear. Perhaps it is related problem #30