When reading on paper-books I usually use a ruler to keep reading focused on the right line.
On computer screens this is a bit harder to do: normally I have to use the upper edge of the screen for the same effect.
Recently I came across a nice little feature on Skim and thought this would look very good on DT3:
it is called a reading-bar.
On Skim you have to drag it up and down with the mouse, but IMO it would be more comfortable if it just moved along with the mouse after you activate it.
So, just another suggestion for you guys.