The Config tab's footer sat mid-panel with further rows drawing below the
window's bottom edge. Live-DAT measured: the mounted tab-host root is
authored 300x362 (retail's real default window size), but the Config page
slot underneath keeps its own larger design geometry (298x575 against a
300x600 canvas) until retail's real four-edge UiLayoutPolicy
(UIElement::UpdateForParentSizeChange @0x00462640) shrinks it on the first
ApplyAnchor pass -- verified stable, this part already worked.
The actual bug: UiTemplateListBox.Viewport (the UiScrollablePanel that
hosts + clips every row) is a programmatic C# element seeded at Bind time,
BEFORE the tree's first draw frame -- before the ListBox has ever shrunk.
Its legacy anchor baseline is captured lazily on its own first ApplyAnchor
call, which lands AFTER the ListBox has already shrunk earlier in that same
frame (parent-before-child draw order). That capture measures a negative
bottom margin the stretch math preserves forever: the viewport stayed
locked at its original 560px design height, clipping rows to a bound
retail never actually gave the window on screen.
Fix: force the viewport's anchor capture to happen immediately after
seeding it, while its Width/Height still exactly equal a zero-margin
baseline against the CURRENT (pre-shrink) parent, instead of lazily on the
first draw frame against an already-shrunk parent. This is #372's sequel --
#372 fixed the 0x0 collapse case; this is the "ListBox itself later
shrinks" case #372's own fixture never exercised.
Three new tests (UiTemplateListBoxViewportTests using the live-DAT-measured
298x575/276x560 numbers, plus two ConfigOptionsPageControllerTests against
the real production Bind path and the committed host fixture) all fail
pre-fix, confirmed by temporarily reverting the change. Scoped to
UiTemplateListBox's own viewport; UiScrollablePanel/ApplyAnchor/
ComputeAnchoredRect are untouched, so chat's transcript scrolling and every
other UiScrollablePanel/UiItemList consumer are unaffected.
fix#412
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
c623b57ad3