Comment # 1 on bug 1141308 from Wolfgang Bauer

(In reply to Christoph Resch from comment #0)
> https://cgit.kde.org/kpimtextedit.git/commit/
> ?id=7e5fa67b0dc1f0623f947ffa7ee791d6ecd335e4
> 
> there has been a minor fix to kmail-composer that would be nice to find its
> back into 18.12.3 (OS leap15.1)

That alone wouldn't help much though, it needs
https://cgit.kde.org/kpimtextedit.git/commit/?id=f953a07ee13c3654939fd8922d765245a55f9afb
too... ;-)

I'll have a look if it is possible to backport.