Bright triangles on active tab with TBB 4.0

I downloaded and started Tor Browser Bundle 4.0. Now I encounter bright triangles on each side of the active tab. This occurred on the current version for the first time and neither on 3.6.6. nor on any of the 4.0-alphas.

Tor Browser runs on a Debian box with awesome WM. I see this behavior on  every start of To Browser and with any active tab.