I see you work for Autodesk, so I'll drop these here for you, and hope that they also help others seeking insight into this part of Tumble "problems".
In the docs, if Multi-Gesture Mode Only is set for the trackpad, there are two intriguing oddities, and one bug:
1. Swipe with one-finger for tumble has a slight delay that's quite annoying, and is then overly sensitive. I don't mind the sensitivity, but I don't think most would like it. I can't imagine anyone liking the delay.
2. One finger gesture tumbling occurs around the centre of the viewport, regardless of the mode/states set in the Tumble settings.
And the bug:
After tumbling with the one finger gesture, if you goto tumbling with the mouse, it's also now tumbling around the centre of the viewport, despite the selection remaining constant throughout all of the above, and having previously tumbled around that selection.
Selecting another selection, of any sort, resets the tumble to the new selection.
However, another bug... if the viewport is panned prior to making a new selection, with the mouse, the subsequent tumbling (with the mouse) remains around a fixed position in world space, where it started tumbling from during the finger-swipe gesture tumble. So it's entirely possible to move the tumble pivot right out of the viewport. Which is unlikely to be the desired behaviour of most users, most of the time. It'd be rare to ever want this, even rarer to need it to act like this.
// I don't mind the finger swipe tumble setting the tumble pivot to the middle of the viewport. I think that's a good feature shortcut to the problem of getting back to a middle of viewport tumble from selection tumble. I do think it should stay consistently at that point, in relative terms, throughout subsequent panning.
