Microsoft Pinyin IME are duplicating some inputs

[ZxBing0066]

Details

• Browser and browser version: chrome 80+
• OS version: window 10
• xterm.js version: 4.3.0 - latest

Steps to reproduce

1. A demo here: https://codesandbox.io/s/crimson-frost-s11ts?file=/src/index.js:0-267
2. Use Microsoft Pinyin number input, and then input numbers or symbols

A simple demo

import { Terminal } from 'xterm';
import 'xterm/css/xterm.css';

const term = new Terminal();

term.open(document.getElementById('terminal'));
term.write('Hello from \x1B[1;3;31mxterm.js\x1B[0m $ ')

term.onData(data => {
  console.log(data);
  term.write(data);
});

same to #1527, but it's closed

[kena0ki]

I confirmed this issue also occurs with full-width space as pointed in the issue #1548.
The reason why the input characters are duplicated is that CompositionHelper._handleAnyTextareaChanges and CompositionHelper.compositionend are called from keydown event and compositionend event respectively.

FYI:
In VS Code, compositionend event is not fired at the time the characters input (why?), so CompositionHelper.compositionend is not called and thus the issue does not happen.