Skip size-changed measurement during fullscreen

Hosts determine the container size in fullscreen mode, so size-changed
notifications are not needed. The temporary max-content height override
used for measurement causes visible vertical jitter during animated
fullscreen transitions by briefly reflowing the document on every frame.

Author: mel-anthropic

src/app.ts

@@ -1287,6 +1287,13 @@ export class App extends Protocol<AppRequest, AppNotification, AppResult> {
       scheduled = true;
       requestAnimationFrame(() => {
         scheduled = false;
+
+        // Hosts determine the container size in fullscreen, so size-changed
+        // notifications are not needed.
+        if (this._hostContext?.displayMode === "fullscreen") {
+          return;
+        }
+
         const html = document.documentElement;
 
         // Measure actual content height by temporarily overriding html sizing.