NSTrackingSeparatorToolbarItem adds a white band over the top of the leading split view panes when in full screen mode on macOS 27. That white band appears to have the height of the toolbar.
I filed FB23827858 with a sample project and a video demonstrating the issue. I also wrote about it at:
https://virtualsanity.com/202607/nstrackingseparatortoolbaritem-causes-problems-when-putting-a-window-in-full-screen-on-golden-gate-macos-27/
I am hoping this is addressed before macOS 27 ships.