The required range for the `animDropElementIndex` is between 0 and `this.ariaFocusableItems.length`, inclusive. That maximum value signifies that all tab strip items (and particularly the last one) need to shift out of the way because the user is dragging something to the end of the tab strip.
When dragging past the last tab strip item, `animDropElementIndex == this.ariaFocusableItems.length` and there is no overlapping element. In that scenario, we fall back to setting `dropElement = this.ariaFocusableItems[oldDropElementIndex]` but there is no such element. The `dropElement` is undefined, which ultimately results in calling `Tabbrowser.moveTabsAfter` and appending the dragged tab(s). That happens to be the right result.
However, if you do a short vertical drag, `dropElement` also does not get set, so whatever you were dragging moves to the end of the tab strip. That was bug 1959350 and we tried to fix it by not moving tabs if `dropElement` wasn't set. https://phabricator.services.mozilla.com/D247339
This had the side effect of not making it possible to drop a tab at the end of the tab strip if you were dragging forward. It was still possible to drag forward to the last position, then drag backward slightly to get the front edge of the tab to overlap the last tab in the tab strip, which would allow dropping into the last position.
I think the main thing that would help is to make sure that `dropElement` is always set for valid moves. When dragging past the last tab and there is no overlap, we can fall back to the last non-moving tab strip item.
I needed to swap around how we derive `dropElement` and `newDropElementIndex`. If we fall back to making `dropElement` the last tab, the existing code will try to use `dropElement.elementIndex` as the `newDropElementIndex`, which means the `animDropElementIndex` can never achieve the maximum value that it needs to show all of the tabs moving over. I made it so that we independently determine the `newDropElementIndex` fallback and the `dropElement` fallback without using `newDropElementIndex`.
After this change, `dropElement` should always be a valid non-moving tab strip item if a move should occur. If `dropElement` is undefined, then it's correct that no move should occur.
Differential Revision: https://phabricator.services.mozilla.com/D249715
f982fafede