See w3c/csswg-drafts#6501 to see how I found this.

This line doesn't recompute layout in browsers, because `"height"` is given as a pseudo-element name rather than a property.

The right way to do what it wants is `getComputedStyle(document.body).height`, but given nobody noticed this (and this is generally never needed, manually triggering layout should never be needed to avoid FOUC) it seems better to keep current behavior and just remove the call.