GtkSettings:gtk-primary-button-warps-slider

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 (since GTK+ 3.22.25) clicking the middle mouse button.