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Maintain elements in head/body during hydration #5029

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586d8ea
We should not unmount elements that have effect on styling etc
JoviDeCroock Feb 19, 2026
752f9c2
Fix test
JoviDeCroock Feb 19, 2026
6e73e74
Add test
JoviDeCroock Feb 20, 2026
323d0d9
Add body as well
JoviDeCroock Feb 23, 2026
63200de
Golf
JoviDeCroock Feb 23, 2026
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6 changes: 5 additions & 1 deletion src/diff/index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -555,7 +555,11 @@ function diffElementNodes(
);

// Remove children that are not part of any vnode.
if (excessDomChildren != NULL) {
if (
excessDomChildren != NULL &&
newVNode.type != 'body' &&
newVNode.type != 'head'
) {
for (i = excessDomChildren.length; i--; ) {
removeNode(excessDomChildren[i]);
}
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27 changes: 27 additions & 0 deletions test/browser/hydrate.test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -485,6 +485,33 @@ describe('hydrate()', () => {
expect(getLog()).to.deep.equal(['Comment.remove()', 'Comment.remove()']);
});

it('should preserve existing head children when hydrating document', () => {
document.textContent = '';
document.head.innerHTML =
'<title>Test</title><meta name="viewport" content="width=device-width">';
document.body.innerHTML = '<p>Test</p>';
clearLog();

const App = () => (
<Fragment>
<head>
<title>Test</title>
</head>
<body>
<p>Test</p>
</body>
</Fragment>
);

hydrate(<App />, document);

expect(document.head.querySelector('meta[name="viewport"]')).to.not.equal(
null
);
expect(document.head.querySelector('title').textContent).to.equal('Test');
expect(document.body.innerHTML).to.equal('<p>Test</p>');
});

it('should work with error boundaries', () => {
scratch.innerHTML = '<div>Hello, World!</div>';
class Root extends Component {
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6 changes: 3 additions & 3 deletions test/browser/render.test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1969,6 +1969,7 @@ describe('render()', () => {

it('should work with document', () => {
document.textContent = '';
document.body.textContent = '';
const App = () => (
<Fragment>
<head>
Expand All @@ -1980,9 +1981,8 @@ describe('render()', () => {
</Fragment>
);
render(<App />, document);
expect(document.documentElement.innerHTML.trim()).to.equal(
'<head><title>Test</title></head><body><p>Test</p></body>'
);
expect(document.head.querySelector('title').textContent).to.equal('Test');
expect(document.body.innerHTML.trim()).to.equal('<p>Test</p>');
});

it('should not remount components when replacing a component with a falsy value in-between', () => {
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