Hello.
I also ran across this issue. The cause seems to be that the task lisk pane at the left only has a horizontal scroll bar, so if the cursor is over that pane and I move the mouse wheel (or slide two fingers on the touchpad), the list moves horizontally.
If I move the cursor over to the right side of the window, where there is a vertical scroll bar and move the wheel while the cursor is over that bar, then the task list does scroll vertically. Unfortunately having to move the cursor to the far right just to scroll vertically is inefficient. Hope the developer will add a (very thin or invisible) vertical scroll bar to the task list pane for the Gantt view.
Currently running version 1.9.8
Thanks