Not sure what is going on here, but if you paste the following line of code into the editor, the browser freezes (reproducible in Chrome 15.0.874.121 and Firefox 3.6.20):

background-image:url(../../images/topnav_left_active.png);

Happens only with the new 4.0 editor. See attached screenshot of script error in Firefox when attempting to paste the code as well.

Workaround

Insert this line of code via Wiki Markup:

{code}
background-image:url(../../images/topnav_left_active.png);
{code}