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.)
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?