Make 'fit-frame-to-buffer' work around size hints (Bug#74866)

* lisp/window.el (fit-frame-to-buffer-1): When
'frame-resize-pixelwise' is nil, round up requested sizes to avoid
that lines get wrapped (Bug#74866).
* doc/lispref/windows.texi (Resizing Windows): Mention that with
size hints one may have to set 'frame-resize-pixelwise' to make
'fit-frame-to-buffer' fit the buffer exactly.

2 files changed, 23 insertions, 1 deletions

diff --git a/doc/lispref/windows.texi b/doc/lispref/windows.texi
index adc294e4c99..3ff78b599de 100644
--- a/doc/lispref/windows.texi
+++ b/doc/lispref/windows.texi
@@ -1166,7 +1166,9 @@ frame to its buffer using the command @code{fit-frame-to-buffer}.
 This command adjusts the size of @var{frame} to display the contents of
 its buffer exactly.  @var{frame} can be any live frame and defaults to
 the selected one.  Fitting is done only if @var{frame}'s root window is
-live.
+a live window.  On window systems that use size hints, exact fitting can
+be often achieved if and only if @code{frame-resize-pixelwise}
+(@pxref{Frame Size}) is non-@code{nil}.
 
 The arguments @var{max-height}, @var{min-height}, @var{max-width} and
 @var{min-width}, if non-@code{nil}, specify bounds on the new body size
diff --git a/lisp/window.el b/lisp/window.el
index e9d57652ec6..cd19fd73849 100644
--- a/lisp/window.el
+++ b/lisp/window.el
@@ -9921,6 +9921,26 @@ for `fit-frame-to-buffer'."
             ;; Move frame down.
             (setq top top-margin)))))
       ;; Apply our changes.
+      (unless frame-resize-pixelwise
+        ;; When 'frame-resize-pixelwise' is nil, a frame cannot be
+        ;; necessarily fit completely even if the window's calculated
+        ;; width and height are integral multiples of the frame's
+        ;; character width and height.  The size hints Emacs produces
+        ;; are inept to handle that when the combined sizes of the
+        ;; frame's fringes, scroll bar and internal border are not an
+        ;; integral multiple of the frame's character width (Bug#74866).
+        ;; Consequently, the window manager will round sizes down and
+        ;; this may cause lines getting wrapped.  To avoid that, round
+        ;; sizes up here which will, however, leave a blank space at the
+        ;; end of the longest line(s).
+        (setq text-minus-body-width
+              (+ text-minus-body-width
+                 (- char-width
+                    (% text-minus-body-width char-width))))
+        (setq text-minus-body-height
+              (+ text-minus-body-height
+                 (- char-height
+                    (% text-minus-body-height char-height)))))
       (setq text-width
             (if width
                 (+ width text-minus-body-width)