Editing message with _ to add a mention prepends \ in the plain text

[progval]

Steps to reproduce

1. From https://riot.im/app/ or https://app.element.io/ join a room bridge to IRC
2. Type abc_def
3. Edit the message to add a mention (@val:e2e.zone in the example below)

Outcome

What did you expect?

The edit should send * val: abc_def to IRC

What happened instead?

The edit sends * val: abc\_def to IRC

This is because Element added backslashes in the edit box (escapes the underscore), which end up in the plain text field of the event, but not in the HTML field

Here is the full event:

{
  "content": {
    "body": " * val: abc\\_def",
    "format": "org.matrix.custom.html",
    "formatted_body": " * <a href=\"https://matrix.to/#/@val:e2e.zone\">val</a>: abc_def",
    "m.new_content": {
      "body": "val: abc\\_def",
      "format": "org.matrix.custom.html",
      "formatted_body": "<a href=\"https://matrix.to/#/@val:e2e.zone\">val</a>: abc_def",
      "msgtype": "m.text",
      "org.matrix.msc1767.message": [
        {
          "body": "val: abc\\_def",
          "mimetype": "text/plain"
        },
        {
          "body": "<a href=\"https://matrix.to/#/@val:e2e.zone\">val</a>: abc_def",
          "mimetype": "text/html"
        }
      ]
    },
    "m.relates_to": {
      "event_id": "$XBMc28MHS-DXNdlOG-sbgA-mGAyNPjdyOiE1wZVrAOY",
      "rel_type": "m.replace"
    },
    "msgtype": "m.text",
    "org.matrix.msc1767.message": [
      {
        "body": " * val: abc\\_def",
        "mimetype": "text/plain"
      },
      {
        "body": " * <a href=\"https://matrix.to/#/@val:e2e.zone\">val</a>: abc_def",
        "mimetype": "text/html"
      }
    ]
  },
  "origin_server_ts": 1649949612868,
  "sender": "@pinkie:matrix.org",
  "type": "m.room.message",
  "unsigned": {},
  "event_id": "$oMBhZghlvtAQSb75w1G5rFAJcEkpHCyDKkZqAy3KSq4",
  "room_id": "!sbzvsZTdOjEpyDRhiE:libera.chat"
}

Operating system

Debian 11

Browser information

91.8.0esr

URL for webapp

both https://riot.im/app/ and https://app.element.io/

Application version

1.10.10; this does not seem to happen on 1.10.1

Homeserver

matrix.org

Will you send logs?

Yes

[t3chguy]

Closing in favour of #22456