[resize‑observer] Why does `ResizeObserver` default to `"content‑box"`?

[ExE-Boss]

It’s been agreed upon that defaulting to "content‑box" is a legacy mistake, so what’s the rationale behind having a new API default to "content‑box"?

[dlibby-]

At this point we can probably chalk this up to another legacy mistake - the default was chosen for compatibility with the legacy ResizeObserverEntry's contentRect property, which originally was the only observable box/size/rect.

Basically callers that don't pass a box may be relying on observing the content-box and we need to preserve this behavior.