NSSplitView screen artefacts

Question

I'm buildng an app for macOS with an NSSplitView. In the top frame of the splitview, there is another NSSplitView, this time vertical, to make a three-pane window. (Two frames across the top, one frame across the bottom, a bit like FCP etc.)

It looks like this at launch:

However if I resize the app window, or move any of the dividers, then the original divider positions stay drawn on screen. Like this:

How can I remove these screen artefacts?

Answer 1

FIXED. I simply sub-classed NSSPlitView, and set the class-type in the inspector to my new sub-class. This got rid of the screen artefacts. I have no idea why this works.