The splitter between two stacked or side-by-side panes, controlling one pane's size in pixels as a focusable separator with aria-valuenow. Drag with pointer capture (live onValueChange, one onValueCommitted on release or a lost capture), step it with the arrow keys, Shift or Page Up/Down for a large step, Home and End for the bounds, and collapse or restore the pane with Enter or a double-click. The key and drag maths sit in a pure core beside it.
pnpm dlx shadcn@latest add "https://pluginui.com/r/core-resize-handle.json"