Changelog - 2026-11-03

useElementSize follows any element, rounds and says when it measured, and FakeResizeObserver tests it

New Feature

In one line. useElementSize() with no ref returns a ref to give to the element, so it follows an element that mounts later or is replaced. It takes round, it returns measured, and FakeResizeObserver in @venizia/ardor/testing lets a test deliver a size.

The example is in useElementSize.

What changed

• A callback ref. Leave out the ref option and the hook returns ref. Its identity never changes. When the element mounts later, is replaced or is removed, the hook observes the new one and starts again from 0 by 0. A ref object still works as before.
• round. round: true rounds width and height to whole pixels, so a size that moves by a fraction does not render again. The default is false.
• measured. It is false until the first size is read, and also on a host with no ResizeObserver. An element measured at 0 by 0 has measured: true, so you can tell it from one not read yet.
• FakeResizeObserver. FakeResizeObserver.install() puts a fake on globalThis.ResizeObserver and returns a function that puts the previous one back. FakeResizeObserver.flush({ element, width, height }) delivers a size to every observer of that element. happy-dom's own observe() does nothing, so before this a test could not give the hook a size.

Who is affected

• An app that compares the whole return of useElementSize with toEqual({ width, height }). The return now has measured. Read width and height, or add measured to the expectation.
• All other apps. No action needed. The new options and the new field are additive.

See Hooks and Fake the browser APIs.