Shift key should invert scroll direction.

[ReggieLJ]

Using the standard setting, holding the shift key while scrolling in the timeline will cause the timeline to scroll horizontally(right & left) instead of vertically(up & down). But if you change the default behavior on the "Timeline mouse scrolling" setting in the timeline preferences window, the shift key has no effect. 

Presumably the shift key currently only causes the timeline to scroll horizontally, instead it should invert whatever the current default scroll behavior is set to. So if I have scrolling set to horizontal, holding shift should change it to vertical.

[Ann Bens · Accepted solution]

Its not the shift key you should be using but rather CTRL. Then it works as advertised.

[ReggieLJ]

Ah, this is good to know! Thanks so much Ann! However, you'll notice shift has the same effect in the standard "horizontal" mouse scrolling mode. I don't know why they have two buttons that do the same thing, but the functionality breaks down once you change the default scroll behavior.

[ReggieLJ]

(Also, for anyone else looking, it's the Command key on a mac.)