In Chrome, if you continue scrolling in the ToC panel while it's loading more topic titles, the whole page starts scrolling. When the new topic titles are loaded in the ToC panel, the panel takes over the scroll action again.
I can reproduce this in Chrome, but not in Firefox.
There are user reports from Jethro and Eric M on https://www.mediawiki.org/wiki/Topic:S9vyhwtxgz7z8ucf