Okay, so try entering a game of some kind, and becoming an active player. Then open up a panel that has scrollbars on it, such as F2 or F4. Hover the mouse over the scrollbar, and it will automatically scroll to where you're pointing, even if you aren't clicking on it--in fact, clicking somewhere on the screen is the only way to end that effect.
This only applies to active players in a game, or players who were last in the active list if the game is in intermission. If you're in spectate mode and try this, hovering over a scrollbar won't do anything special, just like if you opened up the menu from the title screen.
It can be especially annoying when you hover on the "update interval" scrollbar and unwittingly drag it to a large value like 60, and then the server list appears unresponsive for a while after that until you look at the bottom of the screen and see what happened, despite having no recollection of setting anything like that.