bug#27830: 26.0.50; Left fringe gets truncated by a pixel in window not sharing that edge with frame

[martin rudalics <rudalics@gmx.at>]

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 > I see two potential homes for this: 21.12 "Scroll Bars" and 21.13
 > "Window Dividers".  We can mention it in both, one a full description,
 > the other just a short comment and a cross-reference.  WDYT?

See the attached patch.

martin

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diff --git a/doc/emacs/frames.texi b/doc/emacs/frames.texi
index b168eff..a01721b 100644
--- a/doc/emacs/frames.texi
+++ b/doc/emacs/frames.texi
@@ -1002,6 +1002,16 @@ Scroll Bars
   The visual appearance of the scroll bars is controlled by the
 @code{scroll-bar} face.

+@cindex vertical border
+  On graphical frames, vertical scroll bars implicitly serve to visually
+separate side-by-side windows.  When vertical scroll bars and window
+dividers (@pxref{Window Dividers}) are both disabled, Emacs separates
+such windows with the help of a one-pixel wide @dfn{vertical border}.
+That border occupies the first pixel column of the window on the right
+and may thus overdraw the leftmost pixels of any glyph displayed there.
+If these pixels convey important information, you can make them visible
+by enabling window dividers with appropriate customizations.
+
 @cindex Horizontal Scroll Bar
 @cindex Horizontal Scroll Bar mode
   On graphical displays with toolkit support, Emacs may also supply a
@@ -1060,7 +1070,12 @@ Window Dividers
 customize the options @code{window-divider-default-bottom-width} and
 @code{window-divider-default-right-width}.

-   For more details about window dividers see @ref{Window Dividers,,
+  When vertical scroll bars are disabled, dividers can be also useful to
+make the first pixel column of a window visible which would be otherwise
+covered by the vertical border used to separate side-by-side windows
+(@pxref{Scroll Bars}).
+
+For more details about window dividers see @ref{Window Dividers,,
 Window Dividers, elisp, The Emacs Lisp Reference Manual}.

 @node Drag and Drop