fix(ui): OP5 review fixes — thumb sync, batched opacity writes, cull register row, tests

Fixes the OP5 (Chat tab) dual-lens review findings against e71e5a96:

- M1 (MUST-FIX): each opacity row's own apply closure now pushes its OWN
  slider's thumb from the post-link truth (bindings.Current*Opacity()),
  mirroring the OP4 binding pattern. Before this, a single-slider drag
  followed by Reset reverted the live value/link but left that slider's
  own thumb stuck at the dragged position.

- S1 (SHOULD-FIX): the Chat tab's two opacity sliders no longer round-trip
  the whole settings.json on every drag MouseMove tick. UiScrollbar gains
  IsDragging + a DragCompleted callback (fires once, at the MouseUp that
  ends an actual thumb drag); the opacity apply closures flush immediately
  when not mid-drag (Reset/Defaults/discrete edits, same as before) and
  defer to DragCompleted otherwise, collapsing dozens of per-tick writes
  into exactly one per drag gesture. Live opacity still applies every tick.

- S2 (SHOULD-FIX): filed register row AP-201 and issue #371 for the
  UiScrollablePanel whole-row-cull-vs-clip divergence the review found
  (predates OP5, made user-visible by OP5's 240-260px filter blocks). Not
  fixed in this round (a renderer-level scissor stack is out of scope
  here) — corrected the OP5 connected-gate script instead so a straddling
  block's disappear-then-reappear-whole is no longer reported as a
  self-sizing regression.

- S3 (SHOULD-FIX): the chatWindowMainFilter round-trip test already
  existed in e71e5a96 (the review missed it scrolling past line 330);
  added the genuinely missing coverage instead — a composed test pinning
  RetailUiRuntime.MountChat's window-0 SettingsStore -> ChatWindowState
  seed (MountChat itself needs live DAT access and isn't unit-testable
  directly).

- N11: ScrollbarLinkage_ModelPointsAtTheChatListBoxScroll now asserts
  through the scoped page-slot lookup (UiElement.FindDescendant) instead
  of the flat layout.FindElement, which passed for the wrong reason given
  the shared scrollbar id 0x10000201 — matches OP6's own scrollbar-linkage
  test pattern.

Also updated ConfigOptionsPageControllerTests' local ChatOptionsPageController
Bindings fake for the new FlushOpacity parameter.

Full Release suite: 13,117 passed / 4 skipped / 0 failed (baseline 13,107/4/0
post-OP6 — 10 tests added, zero skips added, zero failures).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/research/2026-08-11-campaign-op-test-script.md

@@ -387,7 +387,15 @@ chat windows already read when deciding which lines to show.
 2. **Scroll the list.** All six sections are reachable; the block heights
    visibly differ from a flat 100px box — each block is exactly tall enough
    to show all of its own rows with no clipping and no dead space below the
-   last row (AP-195's self-sizing).
+   last row (AP-195's self-sizing). **Known, registered behavior (AP-201,
+   OP5 review fix S2): the row viewport culls whole rows rather than
+   clipping them (no scissor stack yet — `UiScrollablePanel.cs`), so a
+   240-260px filter block that straddles the visible edge at a given scroll
+   position can vanish ENTIRELY at that exact offset rather than showing a
+   partially-clipped view. Keep scrolling a little further and the block
+   reappears whole. This is the registered cull-vs-clip divergence, not a
+   self-sizing regression — do not report a block's disappear-then-
+   reappear-whole behavior as a bug.**
 3. **Look closely at a row whose mask covers MULTIPLE underlying message
    types** (Gameplay, Combat, Allegiance, or Fellowship — the composite-mask
    rows per the research doc) versus a single-bit row (e.g. "Error" or
@@ -491,8 +499,12 @@ chat windows already read when deciding which lines to show.
 - A filter checkbox whose toggle does not change live message routing in
   the window it belongs to, or that leaks into a DIFFERENT window.
 - Reset/Defaults not behaving as described in 11-12.
-- Any block whose height looks clipped or has a large empty gap below its
-  last row (a self-sizing regression).
+- Any block that is STILL missing rows, or leaves a large empty gap below
+  its last row, once fully scrolled into view (a genuine self-sizing
+  regression). **Not a regression:** a block that disappears entirely at
+  some intermediate scroll position and reappears whole a little further —
+  that is the registered whole-row-cull behavior (AP-201, see step 2), not
+  a clipping bug.
 - Any setting that reverts to default after a full relaunch (a persistence
   regression) — remember this is local-only, so a SERVER-side relog is not
   the right test here (see item 15's note).