diff --git a/packages/super-editor/src/core/presentation-editor/PresentationEditor.ts b/packages/super-editor/src/core/presentation-editor/PresentationEditor.ts
index 67ccc8f61b..ee6bb91e49 100644
--- a/packages/super-editor/src/core/presentation-editor/PresentationEditor.ts
+++ b/packages/super-editor/src/core/presentation-editor/PresentationEditor.ts
@@ -315,6 +315,7 @@ export class PresentationEditor extends EventEmitter {
   #editorListeners: Array<{ event: string; handler: (...args: unknown[]) => void }> = [];
   #scrollHandler: (() => void) | null = null;
   #scrollContainer: Element | Window | null = null;
+  #scrollContainerValidated = false;
   #sectionMetadata: SectionMetadata[] = [];
   #documentMode: 'editing' | 'viewing' | 'suggesting' = 'editing';
   #inputBridge: PresentationInputBridge | null = null;
@@ -3064,6 +3065,60 @@ export class PresentationEditor extends EventEmitter {
     return win;
   }
 
+  /**
+   * Re-validates the detected scroll container after the first layout completes.
+   *
+   * At setup time, #findScrollableAncestor picks the first ancestor with
+   * overflow-y: auto|scroll — but it can't verify the element actually constrains
+   * content height (content isn't laid out yet). A consumer may set overflow:auto
+   * on the SuperDoc container without constraining its height, causing the element
+   * to expand to fit all content instead of scrolling.
+   *
+   * After layout, we can check scrollHeight vs clientHeight. If the detected
+   * container isn't actually scrollable AND it grew beyond the viewport (ruling
+   * out properly constrained containers that simply don't have enough content
+   * yet), we walk further up to find one that actually scrolls, or fall back
+   * to window.
+   */
+  #revalidateScrollContainer(): void {
+    if (this.#scrollContainerValidated) return;
+    this.#scrollContainerValidated = true;
+
+    if (!(this.#scrollContainer instanceof Element)) return;
+    if (this.#scrollContainer.scrollHeight > this.#scrollContainer.clientHeight + 1) return;
+
+    // A properly constrained container (e.g. height:600px; overflow:auto) may
+    // not be overflowing yet if the document is short. Its clientHeight stays
+    // within viewport bounds. Only switch when the container grew beyond the
+    // viewport — a clear sign its height is unconstrained.
+    const win = this.#scrollContainer.ownerDocument?.defaultView;
+    const viewportHeight = win?.innerHeight ?? 0;
+    if (this.#scrollContainer.clientHeight <= viewportHeight) return;
+
+    let el: Element | null = this.#scrollContainer.parentElement;
+    let next: Element | Window | null = win ?? null;
+
+    while (el) {
+      const { overflowY } = getComputedStyle(el);
+      if ((overflowY === 'auto' || overflowY === 'scroll') && el.scrollHeight > el.clientHeight + 1) {
+        next = el;
+        break;
+      }
+      el = el.parentElement;
+    }
+
+    if (!next || next === this.#scrollContainer) return;
+
+    const prev = this.#scrollContainer;
+    prev.removeEventListener('scroll', this.#scrollHandler!);
+    this.#scrollContainer = next;
+
+    if (next instanceof Element) {
+      next.addEventListener('scroll', this.#scrollHandler!, { passive: true });
+    }
+    this.#domPainter?.setScrollContainer?.(next instanceof HTMLElement ? next : null);
+  }
+
   /**
    * Sets up drag and drop handlers for field annotations and image files.
    */
@@ -3798,6 +3853,7 @@ export class PresentationEditor extends EventEmitter {
       this.#epochMapper.onLayoutComplete(layoutEpoch);
       this.#selectionSync.onLayoutComplete(layoutEpoch);
       layoutCompleted = true;
+      this.#revalidateScrollContainer();
       this.#updatePermissionOverlay();
 
       // Reset error state on successful layout