bug#59527: 29.0.50; Face issues in show-paren context overlay

[Eli Zaretskii <eliz@gnu.org>]

> From: Gabriel <gabriel376@hotmail.com>
> Date: Wed, 23 Nov 2022 21:22:02 -0300
> 
> The context overlay (from show-paren) has some face issues.  First, in
> case font-lock was not applied to a previous region of the buffer, the
> overlay will not display the context with font-lock.  Second, the
> overlay will be displayed with the same background as the top-left line
> of the window instead of the original context background.
> 
> Steps:
> 
> 1) emacs -Q (master "b42cd524b46a4f29ef13e9d03be9d3df917f9aa3")
> 
> 2) eval:
> 
> (progn
>     ;; enable overlay context in show-paren
>     (setopt show-paren-context-when-offscreen 'overlay)
> 
>     ;; just to have a different background to reproduce issue #2
>     (add-hook 'c-mode-hook #'whitespace-mode))
> 
> 3) Open file src/xdisp.c from emacs source tree: C-x C-f emacs/src/xdisp.c
> 
> 4) Go to line 13418: M-g M-g 13418
> 
> 5) Go to next char (the context overlay will be displayed): C-f
> 
> Problem 1: the overlay context does not have font-lock, since font-lock
> was not applied to the previous area that contains the context.  As a
> workaround, execute C-M b (to navigate to context and force font-lock
> update) and then C-M-f to navigate back and display again the overlay
> context (with correct font-lock, this time).  I expect the context
> overlay to always be displayed with an updated font-lock.
> 
> Problem 2: the overlay context will be displayed with the same
> background color as the top-left line of the window (yellow, in this
> case, because of whitespace-mode).  I expect the context overlay to
> always show the correct background, regardless of the face in the
> top-left line.

CC'ing Tassilo, who implemented this feature.