QDocument::toPlainText() has the stupid decision to convert nbsp to
spaces in it, which our users obviously hate. Unfortunately, this is
out of our control to fix completely. It is a very deep rabbit hole. :(
Typing non-breaking spaces in the editor now works. Reopening these
files at a later point has these characters remain intact.
What does NOT work is copy-pasting non-breaking spaces. These will end
up looking like normal spaces when you paste them somewhere else, be it
in Manuskript or some other document. In other words: it is impossible
for users to verify whether something is a non-breaking space or an
ordinary one.
I realize that it makes this partial fix meaningless for many. Sorry. :/
Partially fixes issue 738.