Unexpected HTML paste when copying code from Gitea

Originally reported in https://github.com/go-gitea/gitea/issues/22622. To reproduce:

- Open [this source file](https://try.gitea.io/silverwind/symlink-test/src/branch/master/package.json), select all code lines, copy them
- Paste into GitHub comment box
- Pasted content is completely mangled. Paste result below. Expectation would be to only paste the plaintext parts of the clipboard data.

---

```
{
--

}

```

Reproduce the issue by copying the code from the linked Gitea source file and pasting it into a GitHub comment box. Start by tracing the repository's clipboard and paste handling entry point; done means the pasted result contains only the plaintext code rather than mangled HTML.

Written by the indexing model from the issue text.