Proposal: split the right sidebar in both directions (nested panes)

[jeremytenjo]

Goal

Let the right sidebar split down as well as right, and let panes nest — like an editor split view — instead of stopping at two side-by-side panes.

Current behaviour

The strip's split control (⌘\) only splits right, the dock caps at two panes, and dragging a tab only offers left/right targets.

Proposal

• Add a split-down action (⌘⇧\) and accept top/bottom tab drops.
• Allow any pane to split again along either axis, with per-session persisted layouts.
• Keep the shipped tab, terminal, file and browser providers, and keep tab bodies mounted while the tree nests.

A working prototype is available as a source patch (542 tests, live-tested):
https://github.com/jeremytenjo/dsh-plugin-infinite-split-tabs

Examples

2×2 with Files and Start tabs, split both ways:

Dragging a tab to the bottom edge to create a vertical split:

Ask

Would the maintainers consider supporting this in core, or exposing a small extension point so the dock renderer/axis policy can be supplied by a plugin?

Upstream currently disables external PRs, so I'd appreciate guidance on the preferred path. If this is useful to you, please upvote or comment so the team can gauge interest.