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unexpect `class` rendered with merge

See original GitHub issue

I used preact-render-to-string as a server-side rendering tool and rerender on the client-side with merge.

import { h, render, Component } from 'preact';
/** @jsx h */

class Cmp extends Component {
  render() {
    return <p id="cmp" className="my-cmp">Cmp</p>;
  }
}

class App extends Component {
  render() {
    return (<div id="my-app">
      <p id="before">Before</p>
      <Cmp />
      <p id="after-1">After1</p>
      <p id="after-2">After2</p>
      <p id="after-3">After3</p>
    </div>);
  }
}

server-side rendering

import { h } from 'preact';
import render from 'preact-render-to-string';
const ssrHtml = render(<App />);
res.send(`
  <div>Preact</div>
  <div id="container">${ssrHtml}</div>
  <div id="log"></div>
`);

client-side rendering

import { h, render } from 'preact';
render(
  <App />,
  document.getElementById('container'),
  document.getElementById('my-app')
);

Here is the full example on codepen.

As the result comes out, there is an unexpect class on p#after-1: Server-side Rendering:

<div id="my-app"><p id="before">Before</p><p id="cmp">Cmp</p><p id="after-1">After1</p><p id="after-2">After2</p><p id="after-3">After3</p></div>

Meanwhile Client-side Rendering:

<div id="my-app"><p id="before">Before</p><p id="cmp">Cmp</p><p id="after-1" class="">After1</p><p id="after-2">After2</p><p id="after-3">After3</p></div>

And when I debug on Chrome DevTool, it seems p#my-cmp, p#after-1, p#after-2, p#after-3 are all replaced. removeOrphanedChildren was call twice.