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>

src/AcDream.App/UI/UiScrollbar.cs

@@ -35,6 +35,26 @@ public sealed class UiScrollbar : UiElement
     public Action<float>? ScalarChanged { get; set; }
     public bool Horizontal { get; set; }
 
+    /// <summary>True while a thumb drag is in progress (between a thumb-hit
+    /// <c>MouseDown</c>/drag-start and the matching <c>MouseUp</c>). OP5 review
+    /// fix S1, 2026-08-11: lets a consumer distinguish a per-tick drag edit
+    /// (defer any expensive settle work) from a single discrete edit (settle
+    /// immediately) without threading extra state through the scalar-value
+    /// callback.</summary>
+    public bool IsDragging => _draggingThumb;
+
+    /// <summary>
+    /// Fires once, at the <c>MouseUp</c> that ends a thumb drag — never on a
+    /// <c>MouseUp</c> that was not preceded by an actual drag (a bare click that
+    /// only page-scrolled or jumped, or a stray <c>MouseUp</c> with no prior
+    /// <c>MouseDown</c>). OP5 review fix S1: the drag-end seam neither
+    /// <see cref="ScalarChanged"/> (fires on every tick) nor <see cref="Model"/>
+    /// scrolling provided — the Chat tab's opacity sliders use this to flush a
+    /// batched settings write exactly once per drag gesture instead of once per
+    /// <c>MouseMove</c>.
+    /// </summary>
+    public Action? DragCompleted { get; set; }
+
     /// <summary>
     /// Optional fill rendered beneath the scalar thumb. Retail's combat power
     /// control is a horizontal scrollbar containing a meter child; the importer
@@ -421,9 +441,13 @@ public sealed class UiScrollbar : UiElement
             }
 
             case UiEventType.MouseUp:
+            {
+                bool wasDragging = _draggingThumb;
                 _draggingThumb = false;
                 _pressedButton = EndButton.None;
+                if (wasDragging) DragCompleted?.Invoke();
                 return true;
+            }
         }
 
         return false;
@@ -474,9 +498,13 @@ public sealed class UiScrollbar : UiElement
             }
 
             case UiEventType.MouseUp:
+            {
+                bool wasDragging = _draggingThumb;
                 _draggingThumb = false;
                 _pressedButton = EndButton.None;
+                if (wasDragging) DragCompleted?.Invoke();
                 return true;
+            }
         }
         return false;
     }
@@ -513,9 +541,13 @@ public sealed class UiScrollbar : UiElement
             }
 
             case UiEventType.MouseUp:
+            {
+                bool wasDragging = _draggingThumb;
                 _draggingThumb = false;
                 _pressedButton = EndButton.None;
+                if (wasDragging) DragCompleted?.Invoke();
                 return true;
+            }
         }
 
         return false;