Before, a code block would always end with a final newline. The
newline was added unconditionally by `hide_lines`.
When the code block is syntax highlighted by highlight.js, this is not
a problem, no empty line is added for a final trailing `\n` character.
However, when the code block is editable and thus handled by the ACE
editor, a trailing newline _is_ significant. I believe this issue is
most closely described by https://github.com/ajaxorg/ace/issues/2083
in the upstream repository.
The effect of the way ACE handles newlines is that a code block like
<pre>
Some code
</pre>
will create an editor with _two_ lines, not just one.
By trimming trailing whitespace, we ensure that we don’t accidentally
create more lines in the ACE editor than necessary.
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