Browser error“ResizeObserver loop completed with undelivered notifications”

Why the loop happens, and the fix

1. 01

   A callback that resizes what it observes

   “If you don't intend to have an infinite loop, you should make sure your resizing code does not trigger the resize observer callback.” MDN, ResizeObserver: Observation errors

   Setting a width, height or content in the callback changes the size again, which needs another notification in the same frame. Skip the update when nothing changed, or defer it to the next frame.

   let lastWidth = 0;
   const ro = new ResizeObserver(([entry]) => {
     const width = Math.round(entry.contentRect.width);
     if (width === lastWidth) return;
     lastWidth = width;
     requestAnimationFrame(() => setColumns(width));
   });

2. 02

   A real loop that keeps growing

   “Note that this only prevents user-agent lockup, not the infinite loop itself.” MDN, ResizeObserver: Observation errors

   If an element grows every frame and the message repeats nonstop, the browser is only protecting itself. Find the callback that feeds its own size back into the layout.

3. 03

   A library’s observer, with a layout that looks right

   Charts, rich text editors, virtual lists and UI kits use ResizeObserver internally. Update the library first. If the page looks and works right, the message is harmless noise.

4. 04

   A dev overlay or error tracker treating it as a crash

   webpack-dev-server shows window errors in its overlay, and error trackers report them. Filter this one message out rather than hiding all errors.

   // webpack.config.js
   devServer: {
     client: {
       overlay: {
         runtimeErrors: (error) => !/ResizeObserver loop/.test(error.message),
       },
     },
   },
   
   // Sentry
   Sentry.init({ ignoreErrors: [/ResizeObserver loop/] });

Checked against the vendor’s docs on November 8, 2026: MDN: ResizeObserver.

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