Comment 5 for bug 419337

I see this problem now also on e.g. https://edge.launchpad.net/ubuntu/+source/evolution

It seems to also be dependent on the font size set in Firefox. With the default (serif 16) I see the problem. Moving the font size up to 20 then the portlet snaps in to the expected position. But moving it down to e.g. 12 an other problem appears as the left border increases to be nearly in line with the "Overview" label and making the browser show a horizontal scroll bar.

I've also played a little bit with firebug and the CSS (at fontsize "serif 16") and setting "margin-right" to "-40em" for #maincontent (style-3.0.css) seems to fix it (at least for a browser width less than around 2600px).
Perhaps this helps you to find the source of this.