Focusring problem

I’m using NSTableControlMBS instead of a Xojo listbox to get a really fast listbox. Unfortunately, the NSTableControl is drawing a focusring even though it shouldn’t. Christian said that he wasn’t able to find out why this is done. After doing some testing in the really complex window I found something really ugly: when moving the window splitter really fast the focusring seems to lag behind.

Xojo 2017r3, macOS 10.11 and also seen on 10.13.

What could cause this? Are there any declares to get rid of the focusring? Can I force the NSTableControlMBS to update? Could this be a problem of the window splitter that I use (imSplitter of unknown provenance)?