If the value of this setting is
TRUE, clicking the primary button in a
GtkRange trough will move the slider, and hence set the range’s value, to
the point that you clicked.
If it is
FALSE, a primary click will cause the slider/value to move
by the range’s page-size towards the point clicked.
Whichever action you choose for the primary button, the other action will be available by holding Shift and primary-clicking, or clicking the middle mouse button.