feat(ui): port favorite spell bar overflow

Import the retail arrow-only spell bar scrollbar from LayoutDesc, preserve authored end-button extents and HideDisabled behavior in the shared retained widget, and bind each favorite list to its sole horizontal pixel-scroll model. Match retail selection exposure for off-screen spells and pin the behavior with real-fixture conformance tests.

Document the six-slice world-interaction completion program as the active pre-M4 work order.

Co-authored-by: Codex <codex@openai.com>

docs/research/2026-07-23-retail-spellbar-overflow-pseudocode.md

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+# Retail favorite spell-bar overflow pseudocode
+
+This note is the implementation oracle for world-interaction completion Slice
+1. Addresses refer to the September 2013 EoR named retail client. The exact
+element graph and inherited properties were verified against combat LayoutDesc
+`0x21000073` and the existing imported fixture
+`combat_21000073.json`.
+
+## Authored element graph
+
+Each of the eight `SpellCastSubMenu` groups contains:
+
+```text
+favorite group
+  scrollbar 0x100000B5, 685x36
+    decrement button 0x10000071, 23x36
+      Normal   0x06004CDC
+      Rollover 0x06004CDD
+    increment button 0x10000072, 23x36
+      Normal   0x06004CDE
+      Rollover 0x06004CDF
+  item list 0x100000B6, x=23, y=2, 639x32
+
+scrollbar property 0x77 = increment button 0x10000072
+scrollbar property 0x78 = decrement button 0x10000071
+scrollbar property 0x79 = HideDisabled true
+item-list property 0x71 = horizontal scrollbar 0x100000B5
+```
+
+The spell-bar scrollbar has no authored track or thumb. Its 23-pixel buttons
+occupy the space immediately outside the 639-pixel item-list viewport.
+
+## Initialization and ownership
+
+Retail:
+
+- `SpellCastSubMenu::Init @ 0x004C5A90`
+- `UIElement_Scrollable::PostInit @ 0x004744B0`
+- `UIElement_Scrollable::OnSetAttribute @ 0x00474630`
+
+```text
+SpellCastSubMenu.Init(parent, groupId, tabId, tabIndex):
+    group = parent.GetChildRecursive(groupId)
+    spellItemList = group.GetChildRecursive(0x100000B6)
+    register this as the item-list drag handler
+    spellTabElement = parent.GetChildRecursive(tabId)
+
+UIElement_Scrollable.OnSetAttribute(property):
+    if property id == 0x71:
+        horizontalScrollbarId = property element id
+
+UIElement_Scrollable.PostInit():
+    horizontalScrollbar = resolve horizontalScrollbarId
+    register for scrollbar messages
+    UpdateScrollbarSize(horizontal=true)
+```
+
+The controller does not synthesize arrow buttons. LayoutDesc owns them; the
+ordinary scrollable/listbox relationship owns the offset.
+
+## Size, disabled state, and visibility
+
+Retail:
+
+- `UIElement_Scrollable::UpdateScrollbarSize_ @ 0x004741A0`
+- `UIElement_Scrollbar::SetDisabled @ 0x004703D0`
+- `UIElement_Scrollbar::SetHideDisabled @ 0x00470400`
+- `UIElement_Scrollbar::UpdateLayout @ 0x004710D0`
+
+```text
+UpdateScrollbarSize(horizontal):
+    content = scrollableWidth
+    viewport = element.Width
+
+    if content <= viewport:
+        content = viewport
+        scrollOffsetX = 0
+        disabled = true
+    else:
+        disabled = false
+
+    scrollbar.Horizontal = true
+    scrollbar.Disabled = disabled
+    scrollbar.Proportion = min(1, viewport / content)
+
+Scrollbar.UpdateLayout():
+    scrollingArea = own rectangle
+    position decrement button at the start and subtract its authored width
+    position increment button at the end and subtract its authored width
+    update thumb/track geometry, when those children exist
+    Visible = NOT (Disabled AND HideDisabled)
+```
+
+For the favorite bar, disabled plus HideDisabled removes both arrows when every
+favorite/empty cell fits. With overflow, the arrows remain visible and the
+item list clips the horizontally translated cells.
+
+## Scroll delta and selection exposure
+
+Retail:
+
+- `UIElement_Scrollable::HandleScrollbarMessage_ @ 0x00474370`
+- `UIElement_ListBox::InqScrollDelta @ 0x0046DC10`
+- `SpellCastSubMenu::SetSelected @ 0x004C5B00`
+
+```text
+on decrement:
+    delta = -itemList.InqScrollDelta(horizontal, line)
+    set horizontal pixel offset += delta
+
+on increment:
+    delta = +itemList.InqScrollDelta(horizontal, line)
+    set horizontal pixel offset += delta
+
+ItemList.InqScrollDelta(horizontal, line):
+    if column count is nonzero:
+        return min(viewportWidth, scrollableWidth / columnCount)
+    return 0
+
+SpellCastSubMenu.SetSelected(spellId):
+    for each item in list order:
+        item.Selected = item.spellId == spellId
+        if selected:
+            endowmentSelected = false
+            itemList.ScrollToView(itemIndex)
+    selectedSpellId = spellId
+```
+
+The favorite cells are 32 pixels wide, so a line arrow moves exactly 32
+pixels. Programmatic or keyboard selection exposes the selected cell even when
+it was outside the prior viewport.
+
+## Port constraints
+
+- Preserve the imported `0x71`/`0x77`/`0x78` relationships.
+- Preserve each authored button's dimension; do not assume the 16-pixel chat
+  scrollbar button size.
+- Do not fabricate a track/thumb for this arrow-only control.
+- Derive disabled state from the shared model's content and viewport extents.
+- Hide disabled controls from both drawing and hit-testing.
+- Keep every favorite tab's scroll offset independent.