typing "<space><space>" in empty paragraph removes the whitespace

[bantic]

On Chrome, typing two spaces in a row in an empty paragraph causes them to both be removed and the paragraph is rerendered as an empty paragraph.

[bantic]

Does not happen on Safari 9.0.2 or Firefox 42.0 and 43.0.4. Observed on Chrome 48.0.2564.79 beta and 47.0.2526.106 (stable)

[bantic]

This is caused when there is a listener for "DOMSubtreeModified" added to the window (even if its callback is a no-op). It's reproducible in all browsers mentioned above. It originally appeared to be isolated to Chrome because I had an extension that added the listener in question on Chrome.

Adding a DOMSubtreeModified listener changes the batching of mutation events that mobiledoc-kit's MutationObserver sees.

When no such listener is attached and a user types a space at the end of a text node, the browser replaces that space with an   to preserve the appearance of multiple spaces in the UI. If it didn't do this, the multiple spaces in dom would appear as a single space in the UI. The editor-dom renderer already does this (see #68 and #75). The browser fires the mutation observer callback once, with 3 mutations: adding a space, removing the text node, and replacing it with a text node with a trailing  . The mutation observer callback triggers an editor reparse, and at the moment of reparsing the correct number of spaces are in the text node.

When the listener is attached, however, the browser fires the mutation observer once for each mutation, rather than once at the end of all 3 mutations. This means the editor reparse will happen when the text node is blank, which causes it to replace the text node with a <br> cursor placeholder. At that point the final mutation that would have happened (replacing the blank text node with one with the correct spaces and a trailing  ) never happens because the text node has been removed.

To fix this we will add an explicit listener for typing a space (like we already do for <tab>) and handle it declaratively rather than letting the browser shuffle DOM around.