Closes #5359

Issue #5338

Issue #5356

Issue #5179

Keypress should not happen during composing (as spec) but it does on Windows for Korean IMEs

So that they don't consume commas.

Closes #5343

Closes #5333

Closes #5332

Closes #5325

Closes #5320

The merge view will incorrectly show a "Revert chunk" arrow when
the text on both sides of the view are the empty string.

The getChunks() helper is not prepared to deal with an empty diff
array.

The getDiff() helper returns an empty diff array when both strings
to compare are empty, as the only diff item returned by
diff_match_patch, `[0, ""]`, is removed as a result of the diff
entry containing an empty string.

The chosen (trivial) fix is to simply check for the empty diff
array case at the top of getChunks().

Issue #5312

The hint logic of the `fromList` command did not respect the cursor position inside the token.
Now it will list all matching completions for the substring up to the cursor, instead of only the ones matching the complete token. The behavior is now the same as for the regular autocompletion.

Closes #5290

Closes #5316

When using the editor in "application/json" mode, the standard javascript autocompletion would show up once triggered. Although it's deactivated by default and doesn't make sense to enable autocompletion in JSON mode on its own, this change fixes incorrect completions once you combine the default completion with your own custom logic.

Primary motivation was to remove the use of `innerHTML` because the
Firefox extension validator raises a warning about this.

The obvious solution is to use `textContent` instead of `innerHTML`
but I thought it could be useful to have the ability to pick a
custom visual for the scroll-lock icon.

As per MDN documentation, all the supported browsers as per
CodeMirror documentation support `:after`.

Fix for mouse events that get sent to the wrong document when the script is being run in a different frame than the editor. 

Issue  #5287