Move the cursor over a raised box, like the buttons in a customize
buffer.  Parts of the cursor will remain in the box even after the
cursor is moved out of the button.

I was told that this also occurs on macOS.

I've attached a patch that fixes the problem for me, but it hasn't been
tested on macOS.

(BTW, I'm noticing a general lack of interest in the NS port, where
features from the other ports can take months or even years to land,
especially WRT to GNUStep support.  If there's a list of TODOs with the
NS port, please let me know.  I'd be happy to help bring it up-to-spec.)

Thanks.