I was wondering if anyone knows why IB has inexplicable high-light areas on an odd nib here and there ..
The following is an example:

I mean that the region of light in the region marked by me in red ...
There is no view under a divided view, there are no borders that correspond to it, and so far nothing has complained about "unulocal views," etc. What is it?
update: I recently found out when it happened when this weird "area" was always the same size as the rightmost NSView (whether it be built into NSSplitView or only 2 NSViews nearby.
Many thanks
Adrian S