fix #371: straddling rows clip at the viewport edge instead of vanishing whole

Gate-3 screenshot review (user): 'the chat tab looks like it is missing
per window config' — Chat Window 1's header rendered over a void at the
DEFAULT scroll offset because its 260px self-sized filter block
straddled the viewport's bottom edge and UiScrollablePanel hid
straddling rows WHOLE (AP-201's predicted symptom, now user-observed at
scroll position zero, upgrading it from polish to blocking).

By fix time the UI renderer HAD everything needed: UiRenderContext's
clip stack (PushClip/PopClip with rect intersection + per-draw quad
clipping) and UiElement's ClipsChildren hook, already honored by both
the generic draw walk and hit-testing. The fix is therefore exactly the
shape the filing asked for, in the panel itself:
- ClipsChildren => true: children draw and hit-test clipped to the
  viewport rect.
- The layout cull keeps any INTERSECTING row Visible (was: fully-inside
  only), with a half-pixel margin excluding zero-overlap edge rows;
  fully-outside rows stay hidden as the cheap skip.

AP-201 retired in this commit (AP actives 142 -> 141); #371 closed; the
gate script's Chat-tab steps re-written to expect clean edge clipping
and to treat any whole-block vanish as a regression. Pinned by
StraddlingRow_StaysVisible_AndClipsInsteadOfVanishing (the exact gate-3
geometry: header + 260px straddler in a 430px viewport) and
ViewportClipsChildDrawingAndHitTesting (the clipped slice is not
clickable).

Full Release suite: 13,089 passed / 4 skipped / 0 failed.

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

src/AcDream.App/UI/UiScrollablePanel.cs

@@ -7,11 +7,19 @@ namespace AcDream.App.UI;
 
 /// <summary>
 /// Simple vertical viewport for controller-built row lists. It shares the same
-/// pixel scroll model as chat text and item grids, and clips whole rows because
-/// the UI renderer does not have a scissor stack yet.
+/// pixel scroll model as chat text and item grids. A row that straddles the
+/// viewport edge stays visible and is CLIPPED to the viewport (#371 —
+/// <see cref="ClipsChildren"/> routes the draw walk and hit-testing through
+/// <see cref="UiRenderContext.PushClip"/>); before that fix this class hid a
+/// straddling row WHOLE, which made the Chat tab's 260px per-window filter
+/// blocks vanish into a void at the default scroll offset (AP-201's predicted
+/// symptom, user-observed at Campaign OP gate 3).
 /// </summary>
 public sealed class UiScrollablePanel : UiPanel
 {
+    /// <inheritdoc/>
+    protected override bool ClipsChildren => true;
+
     private readonly Dictionary<UiElement, float> _baseTops = new(ReferenceEqualityComparer.Instance);
 
     public UiScrollable Scroll { get; } = new();
@@ -66,7 +74,12 @@ public sealed class UiScrollablePanel : UiPanel
 
             float top = baseTop - Scroll.ScrollY;
             child.Top = top;
-            child.Visible = top >= -0.5f && top + child.Height <= Height + 0.5f;
+            // #371: INTERSECTION, not full containment — a straddling row stays
+            // visible and ClipsChildren trims it to the viewport at draw and
+            // hit-test time. Rows entirely outside stay hidden (cheap skip);
+            // the half-pixel margin excludes zero-overlap rows sitting exactly
+            // on either edge.
+            child.Visible = top + child.Height > 0.5f && top < Height - 0.5f;
         }
     }