Case below assumes a LTR typing.
- I start typing a long sentence in the edit summary, no matter if it is pre-filled or not - my cursor moves while I'm typing, from left to right
- At a moment, my cursor getting closer to the right border of the field
On a non-OOui edit summary
- the text already typed moves to the left
- my cursor to be still visible and I know what am I typing
On a the new OOui edit summary
- the text already typed stays where it is, like blocked by the left border (it blinks a bit, like when you try to force typing)
- my cursor is covered by the counter, I don't know where I am
- the text I type is overflowed on the right, I don't know what am I typing
Tested on fr.wp and mw.org, while logged-in and logged-out.