To reproduce:
- Hard refresh of page.
- Click "Browse Topics".
- Click something in the first 10 (e.g. 8th topic).
- Click topic name (to re-open the TOC).
- Hold TOC downwards scroll bar button, to continue scrolling down TOC.
It may jitter, but it will not actually scroll down.
Tested in Firefox/Iceweasel 31.3.0.