TypeError frame.contentWindow == null in some browsers

[developit]

TypeError frame.contentWindow == null in some browsers

[developit]

Hi @luwes! Sorry for the slow reply, just came across this issue today. If you're still using this library, I'd be happy to merge a PR that adds the onload hook.

[luwes]

frame.contentWindow is not created instantly it seems. something like this fixes it,

frame.onload = () => (frame.contentWindow.onresize = () => handler(element));

[luwes]

An additional issue we experienced is that iframe.onload can actually take a long time on a heavy webpage, even when the iframe is empty (src not set).

So there is an elapsed time there in which the iframe could have resized without calling the handler.
A solution would be to compare width and height of the element at attaching the onload event and when the onload event fires, and call the handler if either width or height was changed.

A very good alternative is https://github.com/que-etc/resize-observer-polyfill if you don't mind a few extra kb's.