NSTrackingSeparatorToolbarItem causes problems when putting a window in full screen on Golden Gate (macOS 27)
July 18, 2026
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 have a sample project that demonstrates the issue. To reproduce it:
- Download and open the sample project. Observe that the main window contains a simple three pane split view. Also observe that the window it creates has a toolbar with an NSTrackingSeparatorToolbarItem. The NSTrackingSeparatorToolbarItem tracks the divider between the second pane and the third pane.
- Run the attached sample project on a Mac running Golden Gate (macOS 27).
- Click the green traffic light button in the upper left corner of the window.
Expected results: I expected a smooth animation. I expected all three panes to be completely visible when in full screen.
Actual results: The animation looks janky near the top of the screen. When the animation is complete, a white band covers the top of the first two panes.
The symptoms change if you repeat entering and exiting full screen mode during the same session.
This is a video of my following these steps:
I filed this as FB23827858 and am hoping it gets attention.