Shortcode double brackets ending on newline breaks post editor

[Clorith]

When showing off a shortcode you for example by use of [[gallery]], the post editor breaks if you do the final ] on a new line.

A quick typo can result in this. By updating or publishing the post in this state the editor appears blank, you can see an extended bit of HTML in Text view, but the visual editor remains blank.

Changes made in the visual editor will not be saved, the post is essentially "broken" at this point.

JS error causing it Uncaught TypeError: Cannot call method 'slice' of undefined found in shortcode.js on Line 40.

[Clorith]

Attempting a solution for this by stopping the attempt at slicing if there is no result.match.

As the bug only appears to affect shortcodes with in-editor rendering, this would allow the user to easily spot it and edit accordingly as preventing this slicing makes the shortcode render (which would be the correct behavior, as it's not a properly escaped shortcode to begin with).

[Clorith]

Also check if the accidental newlines are preceding the shortcode

[Clorith]

qunit tests for catching newlines in escaped shortcodes

[jorbin]

I consolidated the two patches into one and fixed a jshint error in the new unit test.

[Clorith]

I took the combined patch @jorbin made with the additional unit tests and changed the js fix slightly (the original had an awkward double comma I hadn't spotted for the comments).

I think it's better to check if the match is defined once instead of checking it with each if statement as well, it's the little things.

[gcorne]

I spent some time reviewing the existing patches and reading through the code, and I am fairly confident that the underlying issue was what amounts to a couple typos. In 27907.4.diff​, I fixed the typos and adjusted the new tests. I found both the old and the new tests a little tricky to follow because of how they test only result.index, which might be something worth revisiting.

[nacin]

In 28223:

Shortcode JS: Avoid errors when an escaped shortcode includes a newline between brackets.

props gcorne.
props Clorith, jorbin for tests.
see #27907, for trunk.

[nacin]

In 28224:

Shortcode JS: Avoid errors when an escaped shortcode includes a newline between brackets.

Merges [28223] to the 3.9 branch.

props gcorne.
props Clorith, jorbin for tests.
fixes #27907.