fix: preserve content when applying transitions

Sources/OpenSwiftUICore/Animation/Transition/Transition.swift

@@ -153,10 +153,14 @@ package struct ApplyTransitionModifier<TransitionType>: PrimitiveViewModifier, M
         inputs: _ViewInputs,
         body: @escaping (_Graph, _ViewInputs) -> _ViewOutputs
     ) -> _ViewOutputs {
-        .init()
+        TransitionType.makeView(
+            view: _GraphValue(Child(modifier: modifier.value)),
+            inputs: inputs,
+            body: body
+        )
     }
 
-    nonisolated package static func _viewListCount(
+    nonisolated package static func viewListCount(
         inputs: _ViewListCountInputs,
         body: (_ViewListCountInputs) -> Int?
     ) -> Int? {

Tests/OpenSwiftUICoreTests/Animation/Transition/TransitionTests.swift

@@ -0,0 +1,66 @@
+//
+//  TransitionTests.swift
+//  OpenSwiftUICoreTests
+
+import Foundation
+import OpenAttributeGraphShims
+@_spi(ForOpenSwiftUIOnly) @testable import OpenSwiftUICore
+import OpenSwiftUITestsSupport
+import Testing
+
+@MainActor
+@Suite(.tags(.aigc), .disabled(if: attributeGraphVendor == .oag))
+struct TransitionTests {
+    @Test
+    func viewListCountWitnessPreservesBodyCount() {
+        var bodyCalls = 0
+        let count = viewListCount(
+            ApplyTransitionModifier<IdentityTransition>.self,
+            inputs: _ViewListCountInputs(.init(.invalid))
+        ) { _ in
+            bodyCalls += 1
+            return 2
+        }
+        #expect(count == 2)
+        #expect(bodyCalls == 1)
+    }
+
+    @Test(arguments: [TransitionPhase.willAppear, .identity, .didDisappear])
+    func combinedTransitionPreservesContentLayout(phase: TransitionPhase) {
+        checkLayout(
+            transition: MoveTransition(edge: .leading).combined(with: .opacity),
+            phase: phase
+        )
+    }
+
+    @Test(arguments: [TransitionPhase.willAppear, .identity, .didDisappear])
+    func asymmetricTransitionPreservesContentLayout(phase: TransitionPhase) {
+        checkLayout(
+            transition: AsymmetricTransition(
+                insertion: MoveTransition(edge: .leading).combined(with: .opacity),
+                removal: OpacityTransition()
+            ),
+            phase: phase
+        )
+    }
+
+    private func checkLayout<T: Transition>(transition: T, phase: TransitionPhase) {
+        let content = Color.red
+            .frame(width: 32, height: 48)
+            .modifier(ApplyTransitionModifier(transition: transition, phase: phase))
+        Update.perform {
+            let graph = ViewGraph(rootViewType: type(of: content), requestedOutputs: .layout)
+            defer { graph.invalidate() }
+            graph.setRootView(content)
+            #expect(graph.sizeThatFits(.unspecified) == CGSize(width: 32, height: 48))
+        }
+    }
+
+    private func viewListCount<M: ViewModifier>(
+        _ modifier: M.Type,
+        inputs: _ViewListCountInputs,
+        body: (_ViewListCountInputs) -> Int?
+    ) -> Int? {
+        M._viewListCount(inputs: inputs, body: body)
+    }
+}