Avoid “Unspecified Error” in MS Edge when accessing `document.activeE…

[jdalton]

This PR avoids a tricky “Unspecified Error” in MS Edge 14 when accessing
document.activeElement from inside an iframe.

[jdalton]

I removed the function forking because it seemed like overkill.

[marijnh]

Does it still work when you only wrap the first access to document.activeElement in a try? Non-trivial code wrapped in error-swallowing try/catch scares me, since when someone introduces a bug there, we probably won't notice.

[jdalton]

I'll check it out. What's interesting is the old fallback for IE9 didn't have the loop logic.

[jdalton]

@marijnh
Do you know if any of the documents are created with DOMParser or the result of an XHR?

I'm triaging the bug from the MS Edge side and trying to narrow down the Unspecified Error which is a fallback error for when things go totally wonky.

[jdalton]

@marijnh
Sorry for more q's, but when is there a root property on an element?
Is this something CodeMirror adds?

[marijnh]

The loop is to 'break out' of shadow DOMs (which old IE's don't support, hence it was missing from the other implementation)

[jdalton]

@marijnh

Ok I found the issue.

By whatwg spec document.activeElement will be the current focusable element. If non is active it will be document.body. If document.body isn't available then it should be document.documentElement or null (all browsers just return null).

However, in Edge in an iframe, if activeElement is supposed to be document.body and it's not available it will throw Unspecified Error. Out of the iframe it will result in null.

Update:
Oh snao! It turns out it will do that in Edge, IE 11, IE 10, IE9, IE8... I've updated the code comment to describe the issue better.

[marijnh]

Thanks! Merged as 69669e4.

// IE < 10 will throw when accessed while the page is loading or in an iframe.
// IE > 9 and Edge will throw when accessed in an iframe if document.body is unavailable.

Could it be that these are actually the same case? It was observed 'during loading' before, but most likely that was just because no body was available.

[jdalton]

Could it be that these are actually the same case? It was observed 'during loading' before, but most likely that was just because no body was available.

They could be similar.
I wasn't sure if IE < 10 may also error out of an iframe while the page is loaded so I left that bit in.