test: extract the scroll card and document drag-canvas data flow

AgentDeskFixture.swift sat at exactly 400 lines; the scroll card is
fully self-contained (its offset state never leaves the card), so it
moves to FixtureCards.swift as a standalone view with zero bindings,
landing the main file at 374. Harness-facing labels are byte-identical.
DragCanvas gains two intent comments distinguishing the deliberately
empty updateNSView from a forgotten sync.

tests/fixture-app/AgentDeskFixture.swift

@@ -44,8 +44,6 @@ struct ContentView: View {
     @State private var dropStatus = "empty"
     @State private var rowDropStatus = "empty"
     @State private var dragCanvasResult = "idle"
-    // scroll
-    @State private var scrollOffset = 0
     // native AppKit controls (genuinely AX-settable, unlike SwiftUI's)
     @State private var nativeSliderValue = 0.0
     @State private var nativeStepperValue = 0.0
@@ -90,7 +88,7 @@ struct ContentView: View {
         HStack(alignment: .top, spacing: 16) {
             dragCard
             surfacesCard
-            scrollCard
+            ScrollCard()
         }
     }
 
@@ -344,30 +342,6 @@ struct ContentView: View {
         .frame(width: 320, height: 200)
     }
 
-    // MARK: scroll
-
-    private var scrollCard: some View {
-        Card(title: "Scroll") {
-            StatusReadout(name: "scroll-offset", value: String(scrollOffset))
-            ScrollView {
-                VStack(alignment: .leading) {
-                    GeometryReader { geo in
-                        Color.clear.preference(
-                            key: ScrollOffsetKey.self,
-                            value: geo.frame(in: .named("scroll-space")).minY)
-                    }
-                    .frame(height: 0)
-                    ForEach(1...60, id: \.self) { i in
-                        Text("Scroll Row \(i)").accessibilityLabel("scroll-row-\(i)")
-                    }
-                }
-            }
-            .coordinateSpace(name: "scroll-space")
-            .onPreferenceChange(ScrollOffsetKey.self) { scrollOffset = Int(-$0) }
-            .frame(width: 220, height: 160)
-            .accessibilityLabel("scroll-area")
-        }
-    }
 }
 
 @main

tests/fixture-app/FixtureCards.swift

@@ -0,0 +1,33 @@
+import SwiftUI
+
+// Overflow card extracted from AgentDeskFixture.swift to keep that file under
+// the 400-LOC limit. All accessibility labels are preserved byte-for-byte so
+// the E2E harness (tests/e2e/run.sh) continues to resolve them unchanged.
+// The build compiles every .swift file in this directory as one module.
+
+struct ScrollCard: View {
+    @State private var scrollOffset = 0
+
+    var body: some View {
+        Card(title: "Scroll") {
+            StatusReadout(name: "scroll-offset", value: String(scrollOffset))
+            ScrollView {
+                VStack(alignment: .leading) {
+                    GeometryReader { geo in
+                        Color.clear.preference(
+                            key: ScrollOffsetKey.self,
+                            value: geo.frame(in: .named("scroll-space")).minY)
+                    }
+                    .frame(height: 0)
+                    ForEach(1...60, id: \.self) { i in
+                        Text("Scroll Row \(i)").accessibilityLabel("scroll-row-\(i)")
+                    }
+                }
+            }
+            .coordinateSpace(name: "scroll-space")
+            .onPreferenceChange(ScrollOffsetKey.self) { scrollOffset = Int(-$0) }
+            .frame(width: 220, height: 160)
+            .accessibilityLabel("scroll-area")
+        }
+    }
+}

tests/fixture-app/FixtureComponents.swift

@@ -75,10 +75,15 @@ struct DragCanvas: NSViewRepresentable {
     @Binding var result: String
     func makeNSView(context: Context) -> DragCanvasView {
         let v = DragCanvasView()
+        // The closure captures `result` by reference; DragCanvasView calls it
+        // directly on each mouseUp, so updateNSView needs no re-sync.
         v.onDrag = { result = $0 }
         return v
     }
-    func updateNSView(_ view: DragCanvasView, context: Context) {}
+    func updateNSView(_ view: DragCanvasView, context: Context) {
+        // Intentionally empty: gesture results flow *out* through the captured
+        // binding (via onDrag), never *in* from SwiftUI state — nothing to sync.
+    }
 }
 
 final class DragCanvasView: NSView {