Sounds like a duplicate of #1419 and #1600.
Mousepad had [a similar bug](https://bugzilla.xfce.org/show_bug.cgi?id=15799) which I investigated and is explained in https://github.com/shimmerproject/Greybird/pull/248.
TLDR; it's a GTK+ bug that can be and is corrected for in some themes, but not others.