Fixes #166462

Before this change _initSelection would always begin its selection search at index 0, this caused issues when selecting backwards and the children of the selection delegate also contained children. This happens because when we select backwards and always start our selection search at index 0, if our pointer moves backwards from point b to point a and point a is already on index 0 (of a selection container with N selectables), the selection logic will short-circuit early thinking we found the selection, skipping any selectables in between point b and point a.

After this change _initSelection will begin its selection search at the opposite edge if it exists. This fixes the issue because when entering a new selectable, while dragging backwards and updating the end edge, first the selectable will call ensureChildUpdated and synthesize a start edge. Then it processes the end edge event and begins to search for that selection edge, we now start this search at the opposite edge which allows us ensure all items between both edges are visited so we can create a continuous selection.

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