Feature: draggable split container (resizable panes with a drag gutter) #6716

Summary

A built-in draggable split container (resizable panes with a drag gutter between children), for master-detail and multi-pane layouts.

Motivation

Split panes are one of the most common TUI layouts (file managers, editors, dashboards, master-detail consoles). Today Horizontal/Vertical + CSS width/height gives a fixed or proportional split, but there's no first-class way to let the user drag the border between two panes to resize them, nor a keyboard-resizable equivalent.

Concretely: I'm building a master-detail app — a left list rail and a right detail pane — and users expect to drag the divider to rebalance them. Right now that means a reactive width on one pane plus a thin separator widget that handles MouseDown/MouseMove/MouseUp with manual min/max clamping. It works, but it's boilerplate every split-pane app re-implements, and getting the drag hit-testing/clamping right is fiddly.

Suggested API (sketch)

with SplitContainer(orientation="horizontal", initial=("30%", "70%"),
                    min_sizes=(16, 20)):
    yield LeftRail()
    yield DetailPane()

• Draggable gutter between children (mouse), and keyboard-resizable when the gutter/container is focused.
• Queryable/persistable split ratio, so apps can save and restore layout.
• A SplitContainer.Resized message when the split changes.
• Respect per-child min_sizes (and optionally max).

Alternatives considered

• Horizontal + reactive width + a custom separator widget handling mouse events — the current workaround; ~40 lines and easy to get subtly wrong.
• Community splitters exist but there's no official, supported widget.

Happy to help test a design against a real master-detail app if useful.