Fix minor issues with removing left or right fringes

* lisp/window.el (window-max-chars-per-line): Account for
'left-fringe-width' and 'right-fringe-width' variables.

* doc/lispref/windows.texi (Window Sizes): Document the effect on
window text width when setting the width of one or both fringes to
zero.
* doc/emacs/display.texi (Fringes): Document the effect on window
text width when setting the width of one or both fringes to zero.
(Bug#22891)

doc/emacs/display.texi

@@ -1087,6 +1087,23 @@ To disable this, change the variable
 @code{overflow-newline-into-fringe} to @code{nil}; this causes Emacs
 to continue or truncate lines that are exactly as wide as the window.
 
+  If you customize @code{fringe-mode} to remove the fringes on one or
+both sides of the window display, the features that display on the
+fringe are not available.  Indicators of line continuation and
+truncation are an exception: when fringes are not available, Emacs
+uses the leftmost and rightmost character cells to indicate
+continuation and truncation with special ASCII characters, see
+@ref{Continuation Lines}, and @ref{Line Truncation}.  This reduces the
+width available for displaying text on each line, because the
+character cells used for truncation and continuation indicators are
+reserved for that purpose.  Since buffer text can include
+bidirectional text, and thus both left-to-right and right-to-left
+paragraphs (@pxref{Bidirectional Editing}), removing only one of the
+fringes still reserves two character cells, one on each side of the
+window, for truncation and continuation indicators, because these
+indicators are displayed on opposite sides of the window in
+right-to-left paragraphs.
+
 @node Displaying Boundaries
 @section Displaying Boundaries
 

doc/lispref/windows.texi

@@ -605,6 +605,12 @@ exceed its total height as returned by @code{window-total-height}.
 @cindex body width of a window
 The @dfn{body width} of a window is the width of its text area, which
 does not include the scroll bar, fringes, margins or a right divider.
+Note that when one or both fringes are removed (by setting their width
+to zero), the display engine reserves two character cells, one on each
+side of the window, for displaying the continuation and truncation
+glyphs, which leaves 2 columns less for text display.  (The function
+@code{window-max-chars-per-line}, described below, takes this
+peculiarity into account.)
 
 @defun window-body-width &optional window pixelwise
 This function returns the width, in columns, of the body of window

lisp/window.el

@@ -1889,9 +1889,19 @@ the font."
 	   (ncols (/ window-width font-width)))
       (if (and (display-graphic-p)
 	       overflow-newline-into-fringe
-	       (/= (frame-parameter nil 'left-fringe) 0)
+               (not
-	       (/= (frame-parameter nil 'right-fringe) 0))
+                (or (eq left-fringe-width 0)
+                    (and (null left-fringe-width)
+                         (= (frame-parameter nil 'left-fringe) 0))))
+               (not
+                (or (eq right-fringe-width 0)
+                    (and (null right-fringe-width)
+                         (= (frame-parameter nil 'right-fringe) 0)))))
 	  ncols
+        ;; FIXME: This should remove 1 more column when there are no
+        ;; fringes, lines are truncated, and the window is hscrolled,
+        ;; but EOL is not in the view, because then there are 2
+        ;; truncation glyphs, not one.
 	(1- ncols)))))
 
 (defun window-current-scroll-bars (&optional window)