Since the input object might try to read from that on focusing

Issue #4439

It sometimes produces invalid output.

Closes #4410

Closes #4412

Issue #4426

Issue #4417

Closes #4437

Closes #4432

Issue #4413

Adds an option to keep the active line highlighted even when text
inside the line is selected.

so other potential emacs-type modules can use

Closes #4419

Add template/variable definitions, checking, types, additional
keywords and indentation fixes.

Closes #4401

Issue #4307

As per CommonMark (conflicting with markdown.pl, but never
mind markdown.pl)

Closes #4395

when accessing `document.activeElement` from inside an iframe.

So that the code doesn't get confused when backspacing or
deleting across a line.

This is still flaky. Ideally we'd capture backspace as a key event,
but Android Chrome makes that impossible.

Issue #4307

Rather than on read

And do so only after a delay, so that subsequent input
events get a chance to fire.

Because Android, especially Google Keyboard, just doesn't care

So that the selection isn't reset to the start of the element by
div.focus().

As described in http://caniuse.com/#feat=user-select-none and https://developer.mozilla.org/en-US/docs/Web/CSS/user-select