Hi,
In the 3.4 family (QA_3_4) running on my test server, when testing the Designer and clicking on "Show/hide left menu", nothing happens except that my Firefox 6 console complains about a Content Security Policy violation.
On the same server, trying version 3.5 (master) works fine. Both version have in libraries/header_http.inc.php the line that emits a X-Content-Security-Policy header.
In 3.4, if I remove this line, all works fine.