Split View

/ NSSplitView · NSSplitViewItem /

also called split pane, splitter view, multi-pane layout, navigation split view

A split view divides one window region into two or more panes that can resize independently. The line between them is a split-view divider or splitter when it can be dragged, not merely a decorative separator. A sidebar is one specialized pane and can collapse while the remaining pane expands.

If you called it…

“the window divided into two resizable panels”“the line between the two panes that I drag”“the divider that resizes the sidebar”“the left panel that collapses into the edge”“the toolbar line that follows the sidebar divider”

…you meant Split View.

Anatomy — every part, named

1
Pane (split-view item)NSSplitViewItem
Each independently sized region on either side of the divider is a pane, represented by an NSSplitViewItem.
2
Split-view divider (splitter)NSSplitView.dividerThickness
“The line between the two panes that I drag” is the split-view divider, also called a splitter.
3
Sidebar collapse behaviorNSSplitViewItem.collapseBehavior
The way a sidebar folds away and yields its width to the next pane is its collapse behavior.
4
Tracking separator toolbar itemNSTrackingSeparatorToolbarItem
The toolbar divider that stays exactly above a moving split-view divider is a tracking separator.

Prompt — paste into your agent

Build a native Split View with NSSplitView and NSSplitViewItem (SwiftUI: NavigationSplitView), using a draggable divider between independently sized panes. Give a sidebar item native collapse behavior and align any toolbar boundary with NSTrackingSeparatorToolbarItem.

Debug prompt — when it misbehaves

Paste this, then describe what you’re seeing — it hands your agent the classic failure modes to rule out first.

Debug my macOS split view (NSSplitView, NSSplitViewController). Rule out: divider position not persisting without autosaveName; holding priorities deciding which pane absorbs window resize — the lowest priority pane grows; delegate min/max constraints fighting Auto Layout constraints on the same panes; canCollapse snapping a pane closed on double-click unexpectedly. The symptom:

In code

The exact names this thing goes by in code — each row is one framework’s word for it. Use the row that matches your project (or paste it into your prompt).

AppKit	`NSSplitView`
AppKit	`NSSplitViewItem`
SwiftUI	`NavigationSplitView`
AppKit	`NSSplitViewController`

If you called it…

Anatomy — every part, named

Prompt — paste into your agent

Debug prompt — when it misbehaves

In code

See also