Comment 2 for bug 1545118

    (A) A scrollbar is immediately adjacent to a column splitter
is a special case of
    (B) a scrollbar’s touch area overlaps with the touch area for anything else
which is a special case of
    (C) the touch areas for any two things overlap.

In the long run we will need a solution for (C). But for now, here’s my proposal for (B). <https://goo.gl/kKcynn> Unfortunately that document is Canonical-only and what I’ve written is not yet approved. So for now, here’s what I’ve written:
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As an exception to any standard control touch area clash behavior, if any part of a scrollbar thumb’s touch area clashes with the touch area of one or more other controls, then:
 * If you press on that area, neither/no element should receive the press event until one of the following happens.
 * If you release without a substantial drag, the press and release should be passed to the control(s) that aren’t the thumb.
 * If you begin dragging (nearly) vertically, it should be treated as dragging the thumb.
 * If you begin dragging in any other direction, the press event and any future events should be passed to the control(s) that aren’t the thumb.
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