perf(sidebar): coalesce the resize drag to one width update per frame - #131

One sidebar drag does far more work than it needs to. Not a user-visible break on its own — this is churn, not correctness.

Cause

SidebarResizeHandle.tsx handles mousemove raw, with no throttle or rAF coalescing:

const onMouseMove = (ev: MouseEvent) => {
  const delta = ev.clientX - startX;
  onWidthChange(delta);
};

So ~60 width updates a second while dragging. Each one sets sidebarWidth in App.tsx, which relayouts the workspace — which wakes every terminal's ResizeObserver (useTerminal.ts:858), calls fit() and pty.resize(), and makes ConPTY re-emit its visible screen. A one-second drag is sixty rounds of that across every open terminal.

sidebarWidth is also a dependency of the auto-save listener effect and of handleSaveSession, so every intermediate value tears down and re-registers the listener and rebuilds the callback.

Fix

createResizeDrag collects pointer positions and emits at most one delta per frame. stop() flushes a position still waiting for its frame, so the width settles where the cursor was released rather than a frame behind it.

The frame scheduler is injected rather than calling requestAnimationFrame directly. That's the one design decision worth pointing at: it's what makes the coalescing testable in the existing node-environment unit suite, with no DOM and no new test dependency.

App.tsx reads the width through a ref instead of taking it as an effect/callback dependency, so only the settled width matters.

The rAF shape matches what useTerminal.ts already does at :860 and :980.

Behaviour

Unchanged from the user's side — the width tracks the cursor, and dragging below 80px still auto-collapses. The difference is one update per frame instead of one per event.

Verification

• 7 unit tests drive the coalescing with a hand-cranked scheduler: burst-collapses-to-one, next frame emits again, stop() flushes the pending position, stop() cancels the frame, no emit when the pointer never moved, no double-emit.
• npm test → 550 passed. The 3 failures (pty-manager, shell-context-menu) are present identically on unmodified master — both need native Windows.