CodeMirror: HTML attributes values hints not fully operable with a keyboard

[afercia]

To reproduce, add an HTML widget either in the Widgets screen or in the Customizer:

• start typing a HTML tag, for example "<i" for an input field:

• a list of hints appears and it is possible to use the down and up arrow keys to select one of the hints in the list, pressing Enter inserts the tag
• after the input, start typing the "type" attribute followed by a "="
• a list of possible values for the type attribute appears
• press the down arrow key to try to select one of the hints
• as soon as you release the down arrow key (on keyup), the selection moves back to the first hint thus making impossible to select any of the other hints

Worth noting:

• typing a letter after the "=" seems to make selection via arrow keys work again, for example type type=t and selection via keyboard works
• seems the attribute values hints don't work at all in the theme/plugin editors, where typing for example <input type= does nothing
• doesn't seem to happen in the Additional CSS in the Customizer, where all the hints appear to be operable with a keyboard

I'm really not sure if this is related to something specific to the HTML widget or if it's something that should be fixed upstream. Setting the "Widgets" component for now.

[afercia]

Agreed during today's bug-scrub this is something that should be fixed, hopefully by someone more familiar with CodeMirror and the HTML widget. Moving to future release. /cc @westonruter