In stacked tab mode:

* Longpressing a tab and then moving the finger left/right will reorder the tab.
* Touching down briefly on a tab and then moving the finger left/right will cause the stacked tabstrip to scroll.

Both cases are visually identical to the user; until the finger is moved left/right there is no way to tell which of the two operations is about to happen. Consider making a clear visual distinction between the two.