fix(core): make Resizable separator keyboard-operable

[bhamodi]

Summary

Keyboard resizing of Resizable panels was dead code. The focusable control is the outer role="separator" div (ResizeHandle.tsx: aria-valuenow/min/max, tabIndex={isDisabled ? -1 : 0}), but onKeyDown={handleKeyDown} was attached to the inner, absolutely-positioned hit-area child. Keydown events fire on the focused element and bubble up -- they never travel down to a descendant -- so the Arrow / Home / End / Enter handler (handleKeyDown, ResizeHandle.tsx:387-442) never ran for keyboard users. Pointer dragging worked; keyboard operation of the WAI-ARIA window-splitter pattern did not.

This moves onKeyDown onto the focusable separator so the handler actually fires. It is placed after the {...props} spread and composed with any consumer-supplied onKeyDown (props.onKeyDown?.(e) then handleKeyDown(e)). Ordering choice: the sibling handlers (onDoubleClick / onFocus / onBlur) sit before {...props}, so a consumer prop can override them. For onKeyDown that would silently re-break keyboard resizing, so this accessibility-critical handler is placed last and composes rather than replaces -- a consumer onKeyDown still runs, and the resize handler can't be clobbered.

The bug shipped because there was no test file for ResizeHandle / useResizable. This PR adds one.

handleKeyDown (unchanged) supports: ArrowRight / ArrowDown grow, ArrowLeft / ArrowUp shrink (step 10, or 50 with Shift), Home jumps to minSizePx, End jumps to maxSizePx (only when a finite max is set), Enter toggles collapse (only when collapsible). Horizontal handles are RTL-aware. Keyboard resizing is pure state math in useResizable (_onResizeStart seeds a ref from size state; _onResizeMove applies a delta), so it needs no layout measurement and its observable effect is aria-valuenow.

Changes

• Resizable/ResizeHandle.tsx: move onKeyDown from the inner hit-area child to the focusable role="separator" element; place it after {...props} and compose with any consumer onKeyDown. Pointer handlers stay on the hit area.
• Resizable/ResizeHandle.test.tsx: new file (the gap that let the bug ship).

Test plan

• node_modules/.bin/vitest run --root . packages/core/src/Resizable -- 11 pass (ResizeHandle.test.tsx). Coverage: ARIA wiring + orientation, separator focusability, ArrowRight/ArrowLeft (± step), Shift = large step, Home → min, End → max, vertical handle ArrowDown/ArrowUp, Enter collapse when collapsible, disabled ignores input (tabIndex -1), and consumer onKeyDown runs alongside resizing (guards the ordering choice).
• Failing-first confirmed: with the source change stashed (test file kept), the 8 keyboard tests fail (aria-valuenow stays 200) while the 3 non-keyboard tests (ARIA wiring, focusable, disabled) pass -- exactly the signature of a handler that never fires. After the fix, all 11 pass.
• node_modules/.bin/tsc --project packages/core/tsconfig.json --noEmit -- clean.
• node_modules/.bin/eslint on both changed files -- clean.

Notes

Found during an a11y audit of core components; scoped to one fix. It deliberately does not change role="separator" emission for non-resizable handles (a separate finding), the keyboard steps, or any pointer logic. The file header already documented "keyboard support," which is now actually true.

[vercel]

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