Unexpected text deletion on selection related to dead keys

[silverwind]

On macOS with Browsers Firefox 99 or Safari 15 (but not in Chrome 99), there is a bug where Codemirror unexpectedly deletes content when accent characters like ` are present and when the current keyboard layout has that key as a dead key. Disabling the dead key in the keyboard layout eliminates the issue.

Here's an example of entering a text with backtick characters and then using the mouse to select the content via a drag action:

For comparison, a regular <textarea> does not show such issues:

[marijnh]

Attached patch seems to help with this.