User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Firefox/91.0

Steps to reproduce:

1. Filled task description with enough text that scroll bar appeared
2. Moved cursor to first line
3. Pressed enter at beginning (or in middle) of first line

Actual results:

1. The previous line scrolled up (rather than the following scrolling down)

This makes it look like nothing happened if pressing enter at beginning of that line

Expected results:

1. It should have scrolled the following text down one line, just like what happens when pressing enter in any other line than the topmost displayed line.

This didn't happen yet in 78.14.0
For the record, it happens in no other text editor except thunderbird mail composition (for mail composition, it happened already in 78.14.0)
It does not happen in Firefox (when editing a <textarea>)

(N.B. I initially reported as "Problem B" in bug 1767392, but following recommendation in comment 5, I've now made this into a separate bug report)

Still an issue in 91.13.0