Spurious "Draft is being edited in other window" errors

[Moin]

I found some steps to reproduce the error (Not every time, but almost)

1. Click the pencil to edit a post (can be a wiki post but also a normal post you can edit)
2. Don’t edit the post but wait (and maybe scroll, click and mark.
   I’m not totally sure what exactly is needed)
3. Minimize the composer, so it disappears, because nothing was changed.
4. Click the pencil again to edit the post