[Docs] polymer-ui-menu on docs page doesn't seem to be displaying correctly, Chromium and Firefox

[monban]

I think there is a problem on the docs page. Apologies for reporting it here, I couldn't find a separate project page for the web site.

When you expand a category, any categories below don't get pushed down, and the overlap makes it unreadable. Also the scrollbar on the nav bar seems unusable.

Resolution steps attempted:
Reset Chromium with Settings -> advanced -> reset browser settings, problem persists.
Reset Firefox with about:support -> 'Reset Firefox...', problem persists.

I assume the problem is in the page since it shows up on both browsers.

Ubuntu 14.04 trusty
Running xfce with compiz
nVidia 331.49 GPU proprietary (non-free) GPU drivers
Desktop is compositing, but the browsers are not using HW acceleration
kernel 3.13.0-24-generic
Chromium 34.0.1847.116 Ubuntu 14.04 aura (260972)
Firefox 30.0
Screenshots of visual artefact follow

[ebidel]

This should be fixed. Dupe of #541