If you scroll the in game scroll bar until O.A.R.D.I.S box is resting flush on top of the grey bar with the copyright and links, then mouse over Sacred Mines, a secondary scroll bar for both horizontal and vertical appears along the outside edges of the game window.
This happens if you have some of the O.A.R.D.I.S box offscreen, or if you have about half a box width of blank space visible beneath it.
In fact, it seems to happen for the game in general, and I only noticed it when it went away after I scrolled beneath O.A.R.D.I.S and it did not happen anymore.