bug#45116: 26.3; refill-mode doesn't allow me to add text at the end of a paragraph

[Eli Zaretskii]

> From: David Klein <dklein0@gmail.com>
> Date: Tue, 8 Dec 2020 19:13:33 +0200
> Cc: 45116@debbugs.gnu.org
> 
> If I typed in the text
> 
> "The quick brown
> 
> jumped
> 
> over
> "
> 
> and then went up to the end of the first line and type the characters
> '[space]' 'H' 'e' 'l' 'l' 'o'
> 
> I get
> 
> "The quick brownHello"
> 
> instead of
> 
> "The quick brown Hello"
> 
> i.e. zero spaces instead of one. Only excess whitespace should be removed
> and the first space isn't excess.

What logic would you suggest to implement to decide that this space is
it not excess?