[DiapDealer]

I can definitely duplicate on Windows with 2.7.5. The weird thing is that its stopped doing it for a long time, and now it's doing it again. But I can't find any rhyme or reason to it. At one time, the white artifact grew diagonally (down and right), but now it's just growing down.

EDIT: The size of the inspector window seems to affect which directions the artifact grows. The only thing I've found that consistently changes whether the artifact appears at all is when the Windows Display Settings are not at the recommended scaling setting. My guess in anything fractional (125% 150% etc...) might aff3ct it adversely.

Maximizing Sigil also stops it from happening on my Windows 10 testing (much of the time). The mouse highlighting is always a bit wonky though. I'm guessing there's JavaScript calculations about window dimensions that are falling down under certain conditions on Windows.

I've had the artifact appear and not appear in all the above mentioned situation, so nothing is foolproof.