Fix undo inside custom element

[ggodlewski]

While using prosemirror inside Custom Element this line fails.
In such case this.prosemirrorView._root is set to ShadowRoot not Document.
Method createRange exists only in Document.

[dmonad]

If you use dom.doc, you are creating a range from the custom element, which use a different range constructor. It is not immediately obvious to me why this would work. I feel that prosemirrorView._root (using the document from the custom element) is the appropriate choice here.

So I assume that your fix simply disables this feature. Could you please verify?

The issue is probably more complex. I'd like to have a reproducible example first and understand why the solution works appropriately in all cases. Please make sure to test that the PR actually works for custom elements.

[ggodlewski]

prosemirrorView._root is not to proper choice because it's either: null, document or shadowRoot.
It is private property of view. Currently it works only because something else accessed root eariler https://github.com/ProseMirror/prosemirror-view/blob/master/src/index.ts#L349

There is no such thing as different range constructor. The only class that implements createRange is Document.
Range is just a class. You can even initiate it with new Range() https://dom.spec.whatwg.org/#dom-document-createrange

The reason I don't use it is because I also use this plugin in nodejs without DOM api, so I use dom.doc which is already there.