The 2017 wikitext editor becomes almost completely unusable for editing when combined with the [[https://developer.mozilla.org/en-US/docs/Tools/Responsive_Design_Mode | responsive design mode]] of Firefox. That's too odd and seems to be taking back the obvious benefit that it seemed to offer (visualising how the page would look like in mobile, to some extent)
=== Steps to reproduce
# Go to [[https://meta.wikimedia.org/wiki/User:NMorales_(WMF)/Sandbox/IdeaLabHomeMobile | User:NMorales (WMF)/Sandbox/IdeaLabHomeMobile]]
# Edit it as source using the 2017 wikitext editor using Firefox that has the "Responsive design mode" feature
# Do some changes
# Preview the changes
# Switch to the responsive design mode (Ctrl+Shift+M) without exiting the preview
# Exit the mode (Ctrl+Shift+M) without exiting the preview
# Exit the preview
# Try to do some editing.
=== Actual behaviour
The editor doesn't behave as it should (deleting content using backspace doesn't delete the content though the cursor moves, issue of T181816 seems to be re-surfacing, etc. etc.)
=== Expected behaviour
The editor should behave as it usually does.
=== Environment details
| **Browser** | Firefox Nightly 62.0a1 |
| **OS** | Debian GNU/Linux 9 (stretch) |
| **Skin** | Vector (default) |