Complete "Pausing computation-heavy widgets" pattern documentation

Add `IntersectionObserver` pattern for pausing expensive operations
(animations, WebGL, polling) when widgets scroll out of view.

- Add `visibilityBasedPause` example to `patterns.tsx` with play/pause
  callbacks and `onteardown` cleanup
- Document pattern in `patterns.md` with link to MDN docs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/patterns.md

@@ -68,6 +68,12 @@ To persist widget state across conversation reloads (e.g., current page in a PDF
 
 > **Note:** For model-visible state (informing the LLM about what the user is viewing), use {@link app!App.updateModelContext updateModelContext} instead. Widget state persistence is for UI state that should survive page reloads but doesn't need to be seen by the model.
 
+## Pausing computation-heavy widgets when out of view
+
+Widgets with animations, WebGL rendering, or polling can consume significant CPU/GPU even when scrolled out of view. Use [`IntersectionObserver`](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) to pause expensive operations when the widget isn't visible:
+
+{@includeCode ./patterns.tsx#visibilityBasedPause}
+
 ## Lowering perceived latency
 
 Use {@link app!App.ontoolinputpartial ontoolinputpartial} to receive streaming tool arguments as they arrive, allowing you to show a loading preview before the complete input is available.

docs/patterns.tsx

@@ -158,6 +158,36 @@ function persistWidgetState(app: App) {
   //#endregion persistData
 }
 
+/**
+ * Example: Pausing computation-heavy widgets when out of view
+ */
+function visibilityBasedPause(
+  app: App,
+  container: HTMLElement,
+  animation: { play: () => void; pause: () => void },
+) {
+  //#region visibilityBasedPause
+  // Use IntersectionObserver to pause when widget scrolls out of view
+  const observer = new IntersectionObserver((entries) => {
+    entries.forEach((entry) => {
+      if (entry.isIntersecting) {
+        animation.play();
+      } else {
+        animation.pause();
+      }
+    });
+  });
+  observer.observe(container);
+
+  // Clean up when the host tears down the widget
+  app.onteardown = async () => {
+    observer.disconnect();
+    animation.pause();
+    return {};
+  };
+  //#endregion visibilityBasedPause
+}
+
 /**
  * Example: Supporting both iframe & MCP Apps in same binary
  */
@@ -183,5 +213,6 @@ void hostStylingVanillaJs;
 void hostStylingReact;
 void persistWidgetStateServer;
 void persistWidgetState;
+void visibilityBasedPause;
 void iframeAndMcpApps;
 void migrateFromOpenai;