Run getComputedStyle in the same context as the target element

[mckenna]

I work on an embedded JavaScript app. We inject an iframe into the page that we're embedded on and the JavaScript is inserted and run in the context of that iframe so we can run in isolation from the parent page.

This works well, but there's an issue in Firefox, specifically with getComputedStyle(), where, if you try to run this method in the context of an iframe, it always returns null.

I'm looking to introduce Popper into my embedded app, but because of the bug above, Popper can't position my elements because getComputedStyle returns null. Unfortunately, this issue has been open for almost a decade and is unlikely to be fixed any time soon.

Because the elements themselves live on the parent page, we can work around this by getting the window of the document that the element is embedded in, and run getComputedStyle() in that context.

This PR solves our specific use case, and should work exactly as before for any other supported browser.

Let me know what you think.

[FezVrasta]

Thank you @mckenna!

@ryanseddon sorry to bother you, but since you worked on (#607) which is pretty similar, may you review this PR and let me know if anything looks off?

[ryanseddon]

You would not believe this but we had this issue yesterday and tracked it down to this same function and I was about to do a similar PR. So approved!

[mckenna]

Nice one on merging this 👍 — any idea when you plan to do another release?