_fck_bookmark="1"

Recently, I upgraded from CKEditor 3.1 to 3.4 and discovered a new to me 'feature'. I have a customized button that allows for specialized formatting, using a <pre> tag with attributes.

{ element : 'pre', attributes : { 'language' : 'php' } };

This worked previously, until the recent upgrade. Whenever I now markup some text using this button, I end up with extra <span> and _fck_bookmark within - I separated the lines below for clarity.

<pre language="php">
<span _fck_bookmark="1" style="display: none;"> </span>
marked up text
<span _fck_bookmark="1" style="display: none;"> </span>
</pre>

What is this span/bookmark, and how do I disable this behavior?

Many thanks!