Python list widget visually breaks when adding quotes are unbalanced

[jfrost-mo]

When typing into a python_list widget, after adding the first quote of a string, the widget falls back to showing the raw value of the setting before the second quote is typed to close the string.

You see something like ["] while it is unbalanced, which is surprising for users.

This issue afflicts rose 2019 as well.

[oliver-sanders]

If this is a pre-existing issue, it's one to transfer to the metomi/rose repo (this repo is just for the porting).

[jfrost-mo]

Moved to metomi#2812