Chrome (and therefore Electron) on the left, Firefox on the right. Right side shows how it used to look like in Chrome as well.
Not sure if this is a Chrome issue, because this behaviour was not there couple versions back.
@scholtzm which system you use?
Those screenshots were done on Windows, but happens on OSX as well, especially visible if you use ::-webkit-scrollbar:
Before (OSX) / Now (OSX)