A ResizeObserver callback that resizes another observed box no deeper in the DOM raises a loop error

Fact

Within one rendering update, after delivering resize observations the browser goes on to deliver only those of targets deeper in the DOM than the shallowest target it just delivered. When a callback changes the size of an observed element at that depth or above, typically by setting a custom property the other element is sized from, that observation is held over to the next frame and ResizeObserver loop completed with undelivered notifications is reported. A target deeper than the first is delivered in the same update, without error.

Why it matters

Chrome (154) dispatches the error only to window error listeners: nothing reaches the console or the DevTools protocol, so Puppeteer's pageerror never fires and a suite asserting no page errors still passes. Error-reporting scripts do catch it, and the dependent callback runs a frame late.

How to apply

• To act on the size of a box that other observers' callbacks change, observe a deeper element, or use an IntersectionObserver with the box as root: it reports children the box no longer fits whole (for example, lines to drop from a log a shrinking room clips) asynchronously, with no loop limit.
• In a test, catch it in the page: add a window error listener in an init script and assert that it heard nothing.