Drop a pane tab on an edge to split

Drag a pane tab to the edge of another pane and drop it: a new split opens there. Cancel the drag to keep the layout.

How to do it

1. Open a pane tab with View ▸ Panes ▸ New Pane Tab (⌃⌥⌘T), or use one you already have.
2. Drag the tab towards the edge of another pane.
3. Drop it on the edge: a new split opens there with that tab in it.
4. To keep the layout as it was, cancel the drag.

More detail in the guide: Tabs, panes, and workspaces