>>     It makes sense to use the new `vertical-border' face not only
>>     for defining a vertical divider on character terminals, but also
>>     on X for defining the foreground color of the vertical thin line
>>     between windows.
>>
>> That seems natural.
>>
>> Is that line present even when there is a scroll bar?
>
> Yes.

I want also to fix the problem where the vertical border steals
1 pixel from the text area of the right window.  This makes some
letters indistinguishable.  The patch below draws the vertical border
1 pixel left when there are no scroll bars and no left fringes.
These screenshots demonstrate how the vertical border looks
before and after applying the patch: