When I left-click anywhere above or below the thumb, it jumps to that position and starts dragging from there. The expected behavior is to scroll one page up or down.
When I right-click anywhere above or below the thumb, it scrolls one page up or down. That is the behavior I expect from left-clicking, the same way it works on GTK2 and QT applications.
Horizontal scrollbars have the same problem.
The problem is caused by GTK 3, and can be cured by creating or editing a file
[Settings] gtk-primary-button-warps-slider = false