fix(UITextDisplayer): avoid ResizeObserver loop warning (#9722)

UITextDisplayer was triggering:

  "ResizeObserver loop completed with undelivered notifications"

This happened because updateCaptions_() mutates the DOM inside the
ResizeObserver callback, which can cause synchronous layout feedback
loops.

This change:

- Observes videoContainer_ instead of textContainer_
- Defers updateCaptions_() using requestAnimationFrame
- Coalesces multiple resize events using a pendingResize_ flag
- Ensures updates only run when captions are visible

This prevents layout feedback loops while preserving correct caption
re-rendering behavior.

Fixes https://github.com/shaka-project/shaka-player/issues/9721

lib/text/ui_text_displayer.js

@@ -122,13 +122,32 @@ shaka.text.UITextDisplayer = class {
       }
     });
 
+    /** @private {boolean} */
+    this.pendingResize_ = false;
+
     /** @private {ResizeObserver} */
     this.resizeObserver_ = null;
     if ('ResizeObserver' in window) {
       this.resizeObserver_ = new ResizeObserver(() => {
-        this.updateCaptions_(/* forceUpdate= */ true);
+        if (this.pendingResize_) {
+          return;
+        }
+        this.pendingResize_ = true;
+        requestAnimationFrame(() => {
+          this.pendingResize_ = false;
+
+          if (!this.textContainer_) {
+            return;
+          }
+
+          if (this.textContainer_.parentElement && this.isTextVisible_) {
+            this.updateCaptions_(/* forceUpdate= */ true);
+          }
+        });
       });
-      this.resizeObserver_.observe(this.textContainer_);
+      goog.asserts.assert(this.videoContainer_,
+          'this.videoContainer_ must be non-null');
+      this.resizeObserver_.observe(this.videoContainer_);
     }
 
     /** @private {Map<string, !HTMLElement>} */