Fix checkbox and radio hydration (#27401)

Fixes whatever part of https://github.com/facebook/react/issues/26876
and https://github.com/vercel/next.js/issues/49499 that
https://github.com/facebook/react/pull/27394 didn't fix, probably.

From manual tests I believe this behavior brings us back to parity with
latest stable release (18.2.0). It's awkward that we keep the user's
state even for controlled inputs, so the DOM is out of sync with React
state.

Previously the .defaultChecked assignment done in updateInput() was
changing the actual checkedness because the dirty flag wasn't getting
set, meaning that hydrating could change which radio button is checked,
even in the absence of user interaction! Now we go back to always
detaching again.

packages/react-dom-bindings/src/client/ReactDOMInput.js

@@ -297,12 +297,10 @@ export function initInput(
     typeof checkedOrDefault !== 'symbol' &&
     !!checkedOrDefault;
 
-  // The checked property never gets assigned. It must be manually set.
-  // We don't want to do this when hydrating so that existing user input isn't
-  // modified
-  // TODO: I'm pretty sure this is a bug because initialValueTracking won't be
-  // correct for the hydration case then.
-  if (!isHydrating) {
+  if (isHydrating) {
+    // Detach .checked from .defaultChecked but leave user input alone
+    node.checked = node.checked;
+  } else {
     node.checked = !!initialChecked;
   }
 

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

@@ -17,7 +17,11 @@ function emptyFunction() {}
 describe('ReactDOMInput', () => {
   let React;
   let ReactDOM;
+  let ReactDOMClient;
   let ReactDOMServer;
+  let Scheduler;
+  let act;
+  let assertLog;
   let setUntrackedValue;
   let setUntrackedChecked;
   let container;
@@ -87,7 +91,11 @@ describe('ReactDOMInput', () => {
 
     React = require('react');
     ReactDOM = require('react-dom');
+    ReactDOMClient = require('react-dom/client');
     ReactDOMServer = require('react-dom/server');
+    Scheduler = require('scheduler');
+    act = require('internal-test-utils').act;
+    assertLog = require('internal-test-utils').assertLog;
 
     container = document.createElement('div');
     document.body.appendChild(container);
@@ -1235,6 +1243,175 @@ describe('ReactDOMInput', () => {
     assertInputTrackingIsCurrent(container);
   });
 
+  it('should hydrate controlled radio buttons', async () => {
+    function App() {
+      const [current, setCurrent] = React.useState('a');
+      return (
+        <>
+          <input
+            type="radio"
+            name="fruit"
+            checked={current === 'a'}
+            onChange={() => {
+              Scheduler.log('click a');
+              setCurrent('a');
+            }}
+          />
+          <input
+            type="radio"
+            name="fruit"
+            checked={current === 'b'}
+            onChange={() => {
+              Scheduler.log('click b');
+              setCurrent('b');
+            }}
+          />
+          <input
+            type="radio"
+            name="fruit"
+            checked={current === 'c'}
+            onChange={() => {
+              Scheduler.log('click c');
+              // Let's say the user can't pick C
+            }}
+          />
+        </>
+      );
+    }
+    const html = ReactDOMServer.renderToString(<App />);
+    container.innerHTML = html;
+    const [a, b, c] = container.querySelectorAll('input');
+    expect(a.checked).toBe(true);
+    expect(b.checked).toBe(false);
+    expect(c.checked).toBe(false);
+    expect(isCheckedDirty(a)).toBe(false);
+    expect(isCheckedDirty(b)).toBe(false);
+    expect(isCheckedDirty(c)).toBe(false);
+
+    // Click on B before hydrating
+    b.checked = true;
+    expect(isCheckedDirty(a)).toBe(true);
+    expect(isCheckedDirty(b)).toBe(true);
+    expect(isCheckedDirty(c)).toBe(false);
+
+    await act(async () => {
+      ReactDOMClient.hydrateRoot(container, <App />);
+    });
+
+    // Currently, we don't fire onChange when hydrating
+    assertLog([]);
+    // Strangely, we leave `b` checked even though we rendered A with
+    // checked={true} and B with checked={false}. Arguably this is a bug.
+    expect(a.checked).toBe(false);
+    expect(b.checked).toBe(true);
+    expect(c.checked).toBe(false);
+    expect(isCheckedDirty(a)).toBe(true);
+    expect(isCheckedDirty(b)).toBe(true);
+    expect(isCheckedDirty(c)).toBe(true);
+    assertInputTrackingIsCurrent(container);
+
+    // If we click on C now though...
+    await act(async () => {
+      setUntrackedChecked.call(c, true);
+      dispatchEventOnNode(c, 'click');
+    });
+
+    // then since C's onClick doesn't set state, A becomes rechecked.
+    assertLog(['click c']);
+    expect(a.checked).toBe(true);
+    expect(b.checked).toBe(false);
+    expect(c.checked).toBe(false);
+    expect(isCheckedDirty(a)).toBe(true);
+    expect(isCheckedDirty(b)).toBe(true);
+    expect(isCheckedDirty(c)).toBe(true);
+    assertInputTrackingIsCurrent(container);
+
+    // And we can also change to B properly after hydration.
+    await act(async () => {
+      setUntrackedChecked.call(b, true);
+      dispatchEventOnNode(b, 'click');
+    });
+    assertLog(['click b']);
+    expect(a.checked).toBe(false);
+    expect(b.checked).toBe(true);
+    expect(c.checked).toBe(false);
+    expect(isCheckedDirty(a)).toBe(true);
+    expect(isCheckedDirty(b)).toBe(true);
+    expect(isCheckedDirty(c)).toBe(true);
+    assertInputTrackingIsCurrent(container);
+  });
+
+  it('should hydrate uncontrolled radio buttons', async () => {
+    function App() {
+      return (
+        <>
+          <input
+            type="radio"
+            name="fruit"
+            defaultChecked={true}
+            onChange={() => Scheduler.log('click a')}
+          />
+          <input
+            type="radio"
+            name="fruit"
+            defaultChecked={false}
+            onChange={() => Scheduler.log('click b')}
+          />
+          <input
+            type="radio"
+            name="fruit"
+            defaultChecked={false}
+            onChange={() => Scheduler.log('click c')}
+          />
+        </>
+      );
+    }
+    const html = ReactDOMServer.renderToString(<App />);
+    container.innerHTML = html;
+    const [a, b, c] = container.querySelectorAll('input');
+    expect(a.checked).toBe(true);
+    expect(b.checked).toBe(false);
+    expect(c.checked).toBe(false);
+    expect(isCheckedDirty(a)).toBe(false);
+    expect(isCheckedDirty(b)).toBe(false);
+    expect(isCheckedDirty(c)).toBe(false);
+
+    // Click on B before hydrating
+    b.checked = true;
+    expect(isCheckedDirty(a)).toBe(true);
+    expect(isCheckedDirty(b)).toBe(true);
+    expect(isCheckedDirty(c)).toBe(false);
+
+    await act(async () => {
+      ReactDOMClient.hydrateRoot(container, <App />);
+    });
+
+    // Currently, we don't fire onChange when hydrating
+    assertLog([]);
+    expect(a.checked).toBe(false);
+    expect(b.checked).toBe(true);
+    expect(c.checked).toBe(false);
+    expect(isCheckedDirty(a)).toBe(true);
+    expect(isCheckedDirty(b)).toBe(true);
+    expect(isCheckedDirty(c)).toBe(true);
+    assertInputTrackingIsCurrent(container);
+
+    // Click back to A
+    await act(async () => {
+      setUntrackedChecked.call(a, true);
+      dispatchEventOnNode(a, 'click');
+    });
+
+    assertLog(['click a']);
+    expect(a.checked).toBe(true);
+    expect(b.checked).toBe(false);
+    expect(c.checked).toBe(false);
+    expect(isCheckedDirty(a)).toBe(true);
+    expect(isCheckedDirty(b)).toBe(true);
+    expect(isCheckedDirty(c)).toBe(true);
+    assertInputTrackingIsCurrent(container);
+  });
+
   it('should check the correct radio when the selected name moves', () => {
     class App extends React.Component {
       state = {