Works fine for me using Firefox 61.0b14 on Windows 10.

In T198015#4312672, @matmarex wrote:

Works fine for me using Firefox 61.0b14 on Windows 10.

Interesting. I tested this just now on a Firefox 60.0.2 on Windows 10. I could reproduce this consistently. To add to that I could reproduce the issue with the following smaller set of steps.

1. Go to User:NMorales (WMF)/Sandbox/IdeaLabHomeMobile
2. Edit it as source using the 2017 wikitext editor using Firefox that has the "Responsive design mode" feature
3. Do some changes
4. Switch to the responsive design mode (Ctrl+Shift+M)
5. Exit the mode (Ctrl+Shift+M)
6. Try to do some editing.

Additional note: This happens even in safemode.

OK. I think I tracked down the real culprit. It seems to be Syntax highlighting that is causing the issues. I don't face any issues with visual editor when I turn-off Syntax highlighting. I'll update the task correspondingly.

BTW, @matmarex did it work fine with or without syntax highlighting for you?

This also hits syntax-highlighted editing in 2010 WTE on the desktop domain on Firefox Daylight (Firefox 81.1 for mobile) with a version of Android I shouldn't still be running (4 year old phone) but which isn't the issue given the existence of this task.

(I use Timeless, which is why that sentence is the way it is.)

Adding this CSS appears to be a partial fix (at least on my mobile device running Firefox Daylight (Android) 92.1.1:

.mw-editfont-monospace {  
    -webkit-text-size-adjust: none; 
    -moz-text-size-adjust: none;  
    text-size-adjust: none;  
}

This mostly fixes the issue with the cursor not aligning with text at all, but it still breaks if you fully delete a line.

@Eviolite that makes sense. we should probably apply that to all edit surfaces, because the OS randomly modifying our font sizes seems a tad dangerous esp. for syntax highlight.