Draftail on firefox crashes when first input into empty field is Cyrillic

[ElectronicRU]

Issue Summary

When using a RichTextField or a RichTextBlock, the initial empty field crashes if the first input is Cyrillic, only on Firefox. Typing some Latin input, erasing it, and typing a Cyrillic letter also triggers the bug. Draftail instances hosted on draftail.org seem unaffected by this bug.

Steps to Reproduce

https://github.com/ElectronicRU/wagtail-rtf-bug - a wagtail start project with minimal changes, build and run as usual.

1. Start a new project with wagtail start project
2. Edit home.models to include a RichTextField
3. Start the server and open the CMS in FIrefox (no repro in Chromium, my version 123.0 Ubuntu snap).
4. Type а Cyrillic character into the field.

DOMException: Node.removeChild: The node to be removed is not a child of this node
    hl http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    gu http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    unstable_runWithPriority http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    Bi http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    vu http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    ru http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    Ki http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    unstable_runWithPriority http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    Bi http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    Ki http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    qi http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    M http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
    Kt http://localhost:8080/static/wagtailadmin/js/vendor.js?v=0b071504:2
vendor.js:2:145994

Here's a relevant snippet from vendor.js:

          if (5 === o.tag || 6 === o.tag) {
            e: for (var s = e, l = o, c = n, d = l; ; ) if (du(s, d, c), null !== d.child && 4 !== d.tag) d.child.return = d,
            d = d.child;
             else {
              if (d === l) break e;
              for (; null === d.sibling; ) {
                if (null === d.return || d.return === l) break e;
                d = d.return
              }
              d.sibling.return = d.return,
              d = d.sibling
            }
            a ? (
              s = r,
              l = o.stateNode,
              8 === s.nodeType ? s.parentNode.removeChild(l) : s.removeChild(l)
            ) : r.removeChild(o.stateNode)
          }

Technical details

• Python version: 3.11
• Django version: repro on 4.1, 5.0
• Wagtail version: repro on 5.2.3, 6.0
• Browser version: Firefox 123.0

Working on this

I'm not great at debugging JS browser differences, so I'm probably not of much use personally.