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>

tests/AcDream.App.Tests/UI/UiScrollablePanelTests.cs

@@ -23,6 +23,57 @@ public sealed class UiScrollablePanelTests
         Assert.True(panel.Children[2].Visible);
     }
 
+    [Fact]
+    public void StraddlingRow_StaysVisible_AndClipsInsteadOfVanishing()
+    {
+        // #371 (Campaign OP gate 3, the Chat tab's "missing per-window
+        // config"): a tall self-sized block (AP-195's 260px filter blocks)
+        // straddling the viewport's bottom edge used to be hidden WHOLE,
+        // leaving a void under its section header at the default scroll
+        // offset. It must stay Visible (the draw/hit walk clips it via
+        // ClipsChildren) — only rows with zero overlap are hidden.
+        var panel = new UiScrollablePanel { Width = 300, Height = 430, LineHeight = 20 };
+        var header = new UiPanel { Top = 0, Width = 300, Height = 18 };
+        var mainBlock = new UiPanel { Top = 18, Width = 300, Height = 260 };
+        var header2 = new UiPanel { Top = 278, Width = 300, Height = 18 };
+        var straddler = new UiPanel { Top = 296, Width = 300, Height = 260 };  // 296..556 in a 430 viewport
+        var below = new UiPanel { Top = 556, Width = 300, Height = 260 };      // fully outside
+        foreach (var c in new[] { header, mainBlock, header2, straddler, below })
+            panel.AddChild(c);
+
+        panel.LayoutScrollableChildren();
+
+        Assert.True(header.Visible);
+        Assert.True(mainBlock.Visible);
+        Assert.True(header2.Visible);
+        Assert.True(straddler.Visible);   // the pre-fix bug: this was false
+        Assert.False(below.Visible);
+
+        // Scrolling far enough hides the first block entirely and keeps the
+        // straddler fully in view — intersection semantics on both edges.
+        panel.Scroll.SetScrollY(296);
+        panel.LayoutScrollableChildren();
+        Assert.False(mainBlock.Visible);  // now fully above the viewport
+        Assert.True(straddler.Visible);
+    }
+
+    [Fact]
+    public void ViewportClipsChildDrawingAndHitTesting()
+    {
+        // The clip half of #371: ClipsChildren must gate the panel's hit-test
+        // so a straddling row's off-viewport portion cannot be clicked.
+        var root = new UiRoot { Width = 800, Height = 600 };
+        var panel = new UiScrollablePanel { Left = 0, Top = 0, Width = 300, Height = 100, LineHeight = 20 };
+        var row = new UiPanel { Top = 60, Width = 300, Height = 120 };  // 60..180, viewport ends at 100
+        panel.AddChild(row);
+        root.AddChild(panel);
+        panel.LayoutScrollableChildren();
+
+        Assert.True(row.Visible);
+        Assert.Same(row, root.Pick(150, 80));    // visible slice: clickable
+        Assert.NotSame(row, root.Pick(150, 140) ?? panel);  // clipped slice: not the row
+    }
+
     [Fact]
     public void ScrollEvent_MovesByLineHeight()
     {