bug#48257: Align to right doesn't account for window separator in terminal frames

[Kevin Boulain <kevinboulain@gmail.com>]

I believe I have a similar issue (I think this behavior has been
present for a while).
Paste and eval this in the scratch buffer (emacs -Q, running from a terminal):
(global-set-key (kbd "<f10>")
                (lambda ()
                  (interactive)
                  ;; 4 chars margin
                  (setq-local right-margin-width 4)
                  ;; force margin to take effect
                  (set-window-buffer (selected-window) (current-buffer))
                  ;; insert in the margin
                  (insert (propertize " " 'display '((margin
right-margin) "1234") 'default t))
                  ;; split the window, should showcase the issue
                  (split-window-right)))

Then press F10 (doesn't matter where the point is): it will set up a
right margin of width 4, insert "1234" then split the window into two.
The window on the left will only show "123" ("4" is probably hidden
behind the window divider) while the right window correctly shows
"1234".
Some packages try to put information in the right margin, but it gets truncated.