Table and HTML paste handlers both insert content for the same paste event

src/index.tsを読んで、subscribe()がハンドラーをインストールする順序をたどり、その後src/paste-markdown-table.tsとsrc/paste-markdown-html.tsを調べて、貼り付けイベントの動作を確認してください。issueの最小例で二重挿入を再現し、すでにキャンセルされたイベントを各ハンドラーがどう扱うかを確認してください。1回の貼り付けイベントでコンテンツが1回だけ挿入されれば完了です。報告にはテストファイル名が記載されていません。

Summary

A single paste event containing an HTML table and a link can insert the selection twice: once through the table handler and again through the HTML/link handler.

This was reported when copying from BlockNote to GitHub: https://github.com/TypeCellOS/BlockNote/issues/3305. However, the duplication can be reproduced with this package alone and with ordinary text/plain content—no BlockNote dependency or Markdown in the plain-text representation is necessary.

Verification

Reproduced in a browser using the published @github/[email protected] package:

1. The table handler inserts its result and calls preventDefault() and stopPropagation().
2. The HTML handler subsequently runs on the same textarea with event.defaultPrevented === true and inserts another representation of the selection.
3. Skipping handlers when event.defaultPrevented is already true prevents the second insertion.

Both handlers are registered by subscribe(), and the duplication also reproduces using subscribe() instead of installing the two handlers individually.

This verifies the published library's behavior; it is not a separate verification of GitHub.com's currently deployed bundle.

Minimal reproduction

Save the following as an HTML file and open it in a browser with network access. Click Run. The results show the unmodified behavior and a guarded control. The control wraps listener registration only to demonstrate the effect of checking defaultPrevented; it is not intended as the proposed implementation.

<!doctype html>
<meta charset="utf-8">
<button id="run">Run</button>
<pre id="results"></pre>
<script type="module">
  import {installTable, installHTML} from 'https://esm.sh/@github/[email protected]?bundle';

  function run(guard) {
    const field = document.createElement('textarea');
    document.body.append(field);
    field.focus();
    const calls = [];
    const original = field.addEventListener;
    field.addEventListener = function(type, listener, options) {
      if (type !== 'paste') return original.call(this, type, listener, options);
      return original.call(this, type, function(event) {
        calls.push({defaultPreventedAtEntry: event.defaultPrevented, skipped: guard && event.defaultPrevented});
        if (guard && event.defaultPrevented) return;
        listener.call(this, event);
      }, options);
    };
    installTable(field);
    installHTML(field);
    const data = new DataTransfer();
    data.setData('text/html', '<p><a href="https://example.com/test">Test</a></p><table><tr><th>A</th><th>B</th></tr><tr><td>C</td><td>D</td></tr></table>');
    // Ordinary plain text, deliberately NOT Markdown.
    data.setData('text/plain', 'Test\nA\tB\nC\tD');
    field.dispatchEvent(new ClipboardEvent('paste', {
      clipboardData: data,
      bubbles: true,
      cancelable: true
    }));
    const result = {guard, calls, output: field.value};
    field.remove();
    return result;
  }

  document.querySelector('#run').addEventListener('click', function() {
    document.querySelector('#results').textContent = JSON.stringify([run(false), run(true)], null, 2);
  });
</script>

Actual result without guard

Handler entry states:

[
  {"defaultPreventedAtEntry": false, "skipped": false},
  {"defaultPreventedAtEntry": true, "skipped": false}
]

Textarea contents:

<p><a href="https://example.com/test">Test</a></p>

A | B
-- | --
C | D

[Test](https://example.com/test)
A	B
C	D

With the guard enabled, the second handler sees defaultPrevented === true and is skipped, removing the duplicate insertion.

Expected behavior

Only one handler should insert content for a paste event. Handlers should coordinate ownership of the event, for example by respecting event.defaultPrevented before processing an already-handled paste.

stopPropagation() does not stop other listeners on the same element, and preventDefault() does not prevent those listeners from running either.

Relevant source

At commit 86be75f8b746fcfc40cb1a9850f7124980c9253a:

• Handler installation order
• Table handler
• HTML/link handler

Related but separate issues

• https://github.com/github/paste-markdown/issues/96 covers nested Markdown links when text/plain already contains Markdown. This report is specifically about duplicate insertion, which also occurs with unformatted plain text.
• The table handler retaining surrounding HTML is another concern; preventing duplicate insertion alone does not fix that behavior.
• https://github.com/opf/openproject/pull/25964 documents a downstream workaround using data-paste-markdown-skip on tables.