[eric] streaming: mid-gesture deltas ride the 1Hz buffer with a 250ms hand-off flush, so token commits stop landing inside drags (ENG-301)

frontend/src/shared/interactionPriority.ts

@@ -0,0 +1,24 @@
+// While the user is mid-gesture (dragging a card, wheeling the canvas), streamed tokens must not
+// steal frames: each delta lands as a React commit, and under several live agents those commits
+// burst to 50-66ms right through the drag (measured on exp.13, the "still feels glitchy" report).
+// Capture-phase listeners keep a decaying "interacting" stamp; stream dispatch consults it and
+// falls back to the 1Hz buffer during the gesture. Smoothness beats token immediacy for ~a second.
+const DECAY_MS = 350;
+let p_lastInteraction = 0;
+
+export function markInteraction(): void {
+  p_lastInteraction = performance.now();
+}
+
+export function interactionActive(): boolean {
+  return performance.now() - p_lastInteraction < DECAY_MS;
+}
+
+let p_installed = false;
+export function installInteractionListeners(): void {
+  if (p_installed || typeof window === 'undefined') return;
+  p_installed = true;
+  window.addEventListener('wheel', markInteraction, { capture: true, passive: true });
+  window.addEventListener('pointerdown', markInteraction, { capture: true, passive: true });
+  window.addEventListener('pointermove', (e: PointerEvent) => { if (e.buttons) markInteraction(); }, { capture: true, passive: true });
+}