Bug: Copy command failing when selecting more than just the quill editor contents

[conlanpatrek]

There's some strange copy / paste behavior happening with quill editors. If you highlight some content in a readonly editor and also include some content outside of the editor, copy commands fail.

Steps to reproduce

1. Setup a readonly quill editor with some outer text content after.
2. Put some content in the quill editor.
3. Try to highlight both the content of the quill editor and the outer text content.

Expected

The highlighted text is copied

Actual

Nothing is copied, the clipboard remains in it's previous state.

Examples

Copy succeeds

Copy fails

Reproduction

I've created a reproduction in the quill playground

[conlanpatrek]

I see the issue, will submit a PR this evening.

[conlanpatrek]

@luin got a pretty simple bug, repro, and bugfix that's ready for you.

Right now I'm working around this by monkeypatching out the 'onCopyCapture' method on the clipboard module for read-only editors, but it's hacky and I'd love to remove it.