Add ViewTransition event callback unit tests (#36467)

Pulling some basic test coverage out of
https://github.com/facebook/react/pull/36135 as these unit tests are
needed regardless of nested enter/exit work.

packages/react-dom/src/__tests__/ReactDOMViewTransition-test.js

@@ -18,6 +18,7 @@ let act;
 let assertLog;
 let Scheduler;
 let textCache;
+let startTransition;
 
 describe('ReactDOMViewTransition', () => {
   let container;
@@ -31,6 +32,7 @@ describe('ReactDOMViewTransition', () => {
     assertLog = require('internal-test-utils').assertLog;
     Suspense = React.Suspense;
     ViewTransition = React.ViewTransition;
+    startTransition = React.startTransition;
     if (gate(flags => flags.enableSuspenseList)) {
       SuspenseList = React.unstable_SuspenseList;
     }
@@ -176,4 +178,288 @@ describe('ReactDOMViewTransition', () => {
     expect(container.textContent).toContain('Card 2');
     expect(container.textContent).toContain('Card 3');
   });
+
+  describe('ViewTransition event callbacks', () => {
+    let originalGetBoundingClientRect;
+    let originalGetAnimations;
+    let originalAnimate;
+    let originalStartViewTransition;
+
+    beforeEach(() => {
+      // Save originals
+      originalGetBoundingClientRect = Element.prototype.getBoundingClientRect;
+      originalGetAnimations = Element.prototype.getAnimations;
+      originalAnimate = Element.prototype.animate;
+      originalStartViewTransition = document.startViewTransition;
+
+      // Mock CSS.escape if it doesn't exist
+      if (typeof CSS === 'undefined') {
+        global.CSS = {escape: str => str};
+      } else if (!CSS.escape) {
+        CSS.escape = str => str;
+      }
+
+      // Mock document.fonts
+      if (!document.fonts) {
+        Object.defineProperty(document, 'fonts', {
+          value: {status: 'loaded', ready: Promise.resolve()},
+          configurable: true,
+        });
+      }
+
+      // Mock getAnimations on Element.prototype (Web Animations API)
+      Element.prototype.getAnimations = function () {
+        return [];
+      };
+
+      // Mock animate on Element.prototype (Web Animations API)
+      Element.prototype.animate = function () {
+        return {cancel() {}, finished: Promise.resolve()};
+      };
+
+      // Mock getBoundingClientRect to return content-length-based sizes
+      // so that hasInstanceChanged can detect updates when text changes.
+      Element.prototype.getBoundingClientRect = function () {
+        const text = this.textContent || '';
+        const width = text.length * 10 + 10;
+        const height = 20;
+        return new DOMRect(0, 0, width, height);
+      };
+
+      // Mock document.startViewTransition
+      document.startViewTransition = function ({update}) {
+        update();
+        return {
+          ready: Promise.resolve(),
+          finished: Promise.resolve(),
+          skipTransition() {},
+        };
+      };
+    });
+
+    afterEach(() => {
+      Element.prototype.getBoundingClientRect = originalGetBoundingClientRect;
+      Element.prototype.getAnimations = originalGetAnimations;
+      Element.prototype.animate = originalAnimate;
+      if (originalStartViewTransition) {
+        document.startViewTransition = originalStartViewTransition;
+      } else {
+        delete document.startViewTransition;
+      }
+    });
+
+    // @gate enableViewTransition
+    it('fires onEnter when a ViewTransition mounts', async () => {
+      const onEnter = jest.fn();
+      const startViewTransitionSpy = jest.fn(document.startViewTransition);
+      document.startViewTransition = startViewTransitionSpy;
+
+      function App({show}) {
+        if (!show) {
+          return null;
+        }
+        return (
+          <ViewTransition onEnter={onEnter}>
+            <div>Hello</div>
+          </ViewTransition>
+        );
+      }
+
+      const root = ReactDOMClient.createRoot(container);
+
+      // Initial render without the ViewTransition
+      await act(() => {
+        root.render(<App show={false} />);
+      });
+      expect(onEnter).not.toHaveBeenCalled();
+      expect(startViewTransitionSpy).not.toHaveBeenCalled();
+
+      // Mount the ViewTransition inside startTransition
+      await act(() => {
+        startTransition(() => {
+          root.render(<App show={true} />);
+        });
+      });
+
+      expect(startViewTransitionSpy).toHaveBeenCalled();
+      expect(onEnter).toHaveBeenCalledTimes(1);
+    });
+
+    // @gate enableViewTransition
+    it('fires onExit when a ViewTransition unmounts', async () => {
+      const onExit = jest.fn();
+
+      function App({show}) {
+        if (!show) {
+          return null;
+        }
+        return (
+          <ViewTransition onExit={onExit}>
+            <div>Goodbye</div>
+          </ViewTransition>
+        );
+      }
+
+      const root = ReactDOMClient.createRoot(container);
+
+      // Initial render with the ViewTransition
+      await act(() => {
+        startTransition(() => {
+          root.render(<App show={true} />);
+        });
+      });
+      expect(onExit).not.toHaveBeenCalled();
+
+      // Unmount the ViewTransition inside startTransition
+      await act(() => {
+        startTransition(() => {
+          root.render(<App show={false} />);
+        });
+      });
+
+      expect(onExit).toHaveBeenCalledTimes(1);
+    });
+
+    // @gate enableViewTransition
+    it('fires onUpdate when content inside a ViewTransition changes', async () => {
+      const onUpdate = jest.fn();
+      const onEnter = jest.fn();
+
+      function App({text}) {
+        return (
+          <ViewTransition onUpdate={onUpdate} onEnter={onEnter}>
+            <div>{text}</div>
+          </ViewTransition>
+        );
+      }
+
+      const root = ReactDOMClient.createRoot(container);
+
+      // Initial render
+      await act(() => {
+        startTransition(() => {
+          root.render(<App text="Short" />);
+        });
+      });
+
+      onEnter.mockClear();
+      expect(onUpdate).not.toHaveBeenCalled();
+
+      // Update content inside startTransition (different text length
+      // produces different getBoundingClientRect values in our mock)
+      await act(() => {
+        startTransition(() => {
+          root.render(<App text="Much longer content here" />);
+        });
+      });
+
+      expect(onUpdate).toHaveBeenCalledTimes(1);
+      // onEnter should NOT fire on an update
+      expect(onEnter).not.toHaveBeenCalled();
+    });
+
+    // @gate enableViewTransition
+    it('fires onShare for paired named transitions instead of onEnter/onExit', async () => {
+      const onShareA = jest.fn();
+      const onExitA = jest.fn();
+      const onShareB = jest.fn();
+      const onEnterB = jest.fn();
+
+      function App({page}) {
+        if (page === 'a') {
+          return (
+            <ViewTransition
+              key="a"
+              name="hero"
+              onShare={onShareA}
+              onExit={onExitA}>
+              <div>Page A</div>
+            </ViewTransition>
+          );
+        }
+        return (
+          <ViewTransition
+            key="b"
+            name="hero"
+            onShare={onShareB}
+            onEnter={onEnterB}>
+            <div>Page B</div>
+          </ViewTransition>
+        );
+      }
+
+      const root = ReactDOMClient.createRoot(container);
+
+      // Render page A
+      await act(() => {
+        startTransition(() => {
+          root.render(<App page="a" />);
+        });
+      });
+
+      // Clear any enter callbacks from initial mount
+      onShareA.mockClear();
+      onExitA.mockClear();
+      onShareB.mockClear();
+      onEnterB.mockClear();
+
+      // Switch from page A to page B inside startTransition
+      await act(() => {
+        startTransition(() => {
+          root.render(<App page="b" />);
+        });
+      });
+
+      // onShare should fire on the exiting side (page A)
+      expect(onShareA).toHaveBeenCalledTimes(1);
+      // onExit should NOT fire when share takes precedence
+      expect(onExitA).not.toHaveBeenCalled();
+      // onEnter should NOT fire on the entering side when paired
+      expect(onEnterB).not.toHaveBeenCalled();
+    });
+
+    // @gate enableViewTransition
+    it('fires onEnter when Suspense content resolves', async () => {
+      const onEnter = jest.fn();
+
+      function App() {
+        return (
+          <ViewTransition onEnter={onEnter}>
+            <Suspense fallback={<div>Loading...</div>}>
+              <div>
+                <AsyncText text="Loaded" />
+              </div>
+            </Suspense>
+          </ViewTransition>
+        );
+      }
+
+      const root = ReactDOMClient.createRoot(container);
+
+      // Initial render - content suspends
+      await act(() => {
+        startTransition(() => {
+          root.render(<App />);
+        });
+      });
+
+      assertLog(['Suspend! [Loaded]', 'Suspend! [Loaded]']);
+      // onEnter fires for the fallback appearing
+      const enterCallsAfterFallback = onEnter.mock.calls.length;
+      onEnter.mockClear();
+
+      // Resolve the suspended content
+      await act(() => {
+        resolveText('Loaded');
+      });
+      assertLog(['Loaded']);
+
+      expect(container.textContent).toBe('Loaded');
+      // The reveal of the resolved content should trigger enter
+      // (or it may have triggered on the initial fallback mount)
+      expect(
+        onEnter.mock.calls.length + enterCallsAfterFallback,
+      ).toBeGreaterThanOrEqual(1);
+    });
+  });
 });