Greyepoxy/issue 2633

[greyepoxy]

This fixes #2633

Core issue was that selection code was trying to get selection from the top level document and not the container elements document.

With this change I updated the unit tests to run in an iframe which should prevent this issue from re-occurring.

[greyepoxy]

I investigated why its not working in several browsers. Looks like its because some browsers re-define HTMLElement (and other dom functions) on the Window object. So checks like someDomNode instanceof HTMLElement always returns false when someDomNode is in an iframe.

I fixed the two cases of this I found in quill and it resolved a bunch of tests but there is one in parchment that is causing a huge number of failures https://github.com/quilljs/parchment/blob/master/src/registry.ts#L83. Will update it to check against the type defined on the current elements window object.

I will open a merge request over there as well to

[greyepoxy]

I believe this change in combination with slab/parchment#82 should resolve all of the cross browser issues (although I just tested on edge/chrome/firefox since I am on windows)