Widget flickering when using Set Size to enlarge

I’m trying to create a kind of masking effect that follows the cursor. To do this I have created two grey rectangle widgets (MaskTop and MaskBottom) to enlarge as I move the mouse over the white space. The MaskTop works fine, but the MaskBottom flickers for some reason as it changes size and I can’t figure out why.

Couldn’t find any similar forum topic so not sure if someone has crossed this behaviour before or perhaps it is a bug.

Appreciate any help.

Thanks. screenmask.rp (44.8 KB)