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> 返値は、要素に対しての getClientRects() が返す矩形の集合である DOMRect (en-US) オブジェクトです。

https://developer.mozilla.org/ja/docs/Web/API/Element/getBoundingClientRect

> The returned value can be thought of as the union of the rectangles returned by getClientRects() for the element, i.e., the CSS border-boxes associated with the element.

https://developer.mozilla.org/en-US/docs/Web/API/Element/getBoundingClientRect

unionの訳が集合？ キレそう