Need some help with understanding <Scroll> control

[merlight]

I have a plain <Scroll> control (XML source). No inherits="ZO_Scroll", since it only supports vertical scrolling and I want horizontal. No scrollbar.

Here's a picture of two Torchbug windows inspecting tbugTabWindow1Tabs:

tbugTabWindow1Tabs is the scroll control in the yellow box. It has 14 children: ActiveBg provides the dark background for the active tab "6", and children 2..14 are tab labels that were added dynamically -- intentionally too many to fit, you can see "_mouseSomething" is clipped on the right.

What I don't understand is that both extents.horizontal and extents.vertical are 0 (these are values returned by GetScrollExtents). And when I do /script tbugTabWindow1Tabs:SetHorizontalScroll(10), nothing happens. Offsets remain at 0.

I tried playing with SetScrollBounding, and when I set it to SCROLL_BOUNDING_UNBOUND and then SetHorizontalScroll(10), what happens is this: ActiveBg (the first child) is reanchored to the parent with offsetX=-10 and offsetY=-0 (not kidding, negative zero). All other children stay where they are.

Looking at how ZO_ScrollContainer is implemented, I'm beginning to think that the scroll control determines its inner extents solely from its first child, and that scrolling is actually implemented by shifting that child's anchor in the opposite direction.

Also does anyone know what different ScrollBounding values do? So far I've figured that UNBOUND allows you to SetVertical/HorizontalScroll beyond what GetScrollExtents returns. All other values (BOUND, CONTAINED, DEFAULT) seem to prevent that.