In the attached screenshot, we have an NSSplitViewController with three split view items. The first two are viewController split view items, while the third is an inspector split view item.
The NSWindow is configured for full screen content so that the inspector split view items is full height itself.
However, when we enable full height content view, AppKit is automatically adding a tracking separator into the toolbar when we don't want one. (Neither of the two split views are sidebars.)
This tracking separator is out of place, cannot be dragged itself, and is drawn under the center aligned segmented control.
We've tried a multitude of permutations to configure the NSWindow, NSToolbar and NSSplitViewItem but to no avail. Surely we're just missing the magic combo but a lot of the properties appear to be no-ops in macOS 26.
How do we use an NSSplitViewController, in a full screen content window, such that we get a full height inspector but we don't get tracking separators for the main split view divider?
After further debugging, apparently that's not a tracking separator but rather the NSSplitViewControllers actual divider that is showing. This was confusing because NSWindow.titlebarAppearsTransparent = false doesn't seem to hide it. So even when the titlebar is not transparent, some underlying content is being shown.
A solution we found was to use a nested NSSplitViewControllers that uses the safe layout guide for its constraints.
That appears to give a full height inspector view with a "content view" that remains below the window's titlebar.