exactly, same as with a frame... I haven't done much with resizable iframes in a while, but I do remember that at least in the v5-v6 browser era that iframes wouldn't resize properly to match their containers without JS to handle the event, I'm unsure if this is still true, as I haven't really done anything with iframes in nearly a decade now.

aside: used to use/implement something like ajax callbacks with hidden frames and dynamic form posts.

Man, I love JS/Browsers today over pre-2016.