Add a Fantom test for a negative zIndex child in a nested flatten/unflatten swap

Author: pawicao

packages/react-native/src/private/renderer/mounting/__tests__/Mounting-itest.js

@@ -446,6 +446,73 @@ describe('ViewFlattening', () => {
       />,
     );
   });
+
+  test('#58647: child with negative zIndex is kept when its parent flattens and its grandparent unflattens', () => {
+    const root = Fantom.createRoot();
+
+    function render(opacityOnGrandparent: boolean) {
+      Fantom.runTask(() => {
+        root.render(
+          <View nativeID="Q" style={{opacity: 0.5}}>
+            <View style={opacityOnGrandparent ? {opacity: 0.5} : null}>
+              <View style={opacityOnGrandparent ? null : {opacity: 0.5}}>
+                <View nativeID="A" />
+                <View nativeID="K" style={{zIndex: -1}} />
+                <View nativeID="B" />
+              </View>
+            </View>
+          </View>,
+        );
+      });
+    }
+
+    const expectedOutput = (
+      <rn-view nativeID="Q">
+        <rn-view>
+          <rn-view key="0" nativeID="K" />
+          <rn-view key="1" nativeID="A" />
+          <rn-view key="2" nativeID="B" />
+        </rn-view>
+      </rn-view>
+    );
+
+    render(false);
+    root.takeMountingManagerLogs();
+
+    render(true);
+    expect(root.takeMountingManagerLogs()).toEqual([
+      'Remove {type: "View", parentNativeID: "Q", index: 0, nativeID: (N/A)}',
+      'Remove {type: "View", parentNativeID: (N/A), index: 2, nativeID: "B"}',
+      'Remove {type: "View", parentNativeID: (N/A), index: 1, nativeID: "A"}',
+      'Remove {type: "View", parentNativeID: (N/A), index: 0, nativeID: "K"}',
+      'Delete {type: "View", nativeID: (N/A)}',
+      'Create {type: "View", nativeID: (N/A)}',
+      'Insert {type: "View", parentNativeID: "Q", index: 0, nativeID: (N/A)}',
+      'Insert {type: "View", parentNativeID: (N/A), index: 0, nativeID: "K"}',
+      'Insert {type: "View", parentNativeID: (N/A), index: 1, nativeID: "A"}',
+      'Insert {type: "View", parentNativeID: (N/A), index: 2, nativeID: "B"}',
+    ]);
+    expect(root.getRenderedOutput({props: ['nativeID']}).toJSX()).toEqual(
+      expectedOutput,
+    );
+
+    render(false);
+    expect(root.takeMountingManagerLogs()).toEqual([
+      'Remove {type: "View", parentNativeID: (N/A), index: 2, nativeID: "B"}',
+      'Remove {type: "View", parentNativeID: (N/A), index: 1, nativeID: "A"}',
+      'Remove {type: "View", parentNativeID: (N/A), index: 0, nativeID: "K"}',
+      'Remove {type: "View", parentNativeID: "Q", index: 0, nativeID: (N/A)}',
+      'Delete {type: "View", nativeID: (N/A)}',
+      'Create {type: "View", nativeID: (N/A)}',
+      'Insert {type: "View", parentNativeID: (N/A), index: 0, nativeID: "K"}',
+      'Insert {type: "View", parentNativeID: (N/A), index: 1, nativeID: "A"}',
+      'Insert {type: "View", parentNativeID: (N/A), index: 2, nativeID: "B"}',
+      'Insert {type: "View", parentNativeID: "Q", index: 0, nativeID: (N/A)}',
+    ]);
+    expect(root.getRenderedOutput({props: ['nativeID']}).toJSX()).toEqual(
+      expectedOutput,
+    );
+  });
 });
 
 describe('reconciliation of setNativeProps and React commit', () => {