iframe briefly freezes

[KatieWoe]

For phetsims/qa#216. Found on Win 7 Internet Explorer. Not found on other browsers on Win 7 or on Win 10 IE.
When dragging any atom in the atomic interactions iframe, it is possible to drag it off screen. If the atom is "let go of" outside the sim, then there are several seconds where the sim can't be interacted with, before intractability returns. @jbphet thought this sounded like a scenery issue.
https://drive.google.com/file/d/1U_9-pEDQXglWy7SgK_6Aeq_STGLU3hHu/view?usp=sharing

[jbphet]

I didn't realize that this was only in IE when @KatieWoe showed it to me. I don't think it's worth @jonathanolson's time to investigate in that case. Closing.