master 0ff6ee371f0: Fix :align-to 'center' when line numbers are displayed

branch: master
commit 0ff6ee371f03aba3e52469811c7c783e945a03c8
Author: Andrea Alberti <[email protected]>
Commit: Eli Zaretskii <[email protected]>

    Fix :align-to 'center' when line numbers are displayed
    
    When line numbers are shown, 'center' was computed as the center of the
    whole text area, which includes the line-number display, so it landed
    half the line-number width too far to the right.  Center it on the
    editable text area instead.  (Bug#81275)
    * src/xdisp.c (calc_pixel_width_or_height): Correct the adjustment of
    'center' for line-number display, so 'center' is the center of the
    editable part of the text area.
    
    * doc/lispref/display.texi (Pixel Specification): Say that 'center'
    denotes the center of the editable text area.
---
 doc/lispref/display.texi | 4 +++-
 src/xdisp.c              | 7 ++++---
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index ae7d314c795..88654af4b09 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -5701,7 +5701,9 @@ edge, center, or right edge of the text area.  When the window
 displays line numbers, and @code{:align-to} is used in display
 properties of buffer text (as opposed to header line, see below), the
 @code{left} and the @code{center} positions are offset to account for
-the screen space taken by the line-number display.
+the screen space taken by the line-number display.  In particular,
+@code{center} then denotes the center of the editable text area, i.e.@:
+the text area excluding the line-number display.
 
   Any of the above window elements (except @code{text}) can also be
 used with @code{:align-to} to specify that the position is relative to
diff --git a/src/xdisp.c b/src/xdisp.c
index 295a13b99ed..f991adb7d98 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -30506,11 +30506,12 @@ calc_pixel_width_or_height (double *res, struct it *it,
 	  /* 'right': right edge of the text area.  */
 	  if (EQ (prop, Qright))
 	    return OK_ALIGN_TO (window_box_right_offset (it->w, TEXT_AREA));
-	  /* 'center': the center of the text area.  */
+	  /* 'center': the center of the editable text area, i.e. the text
+	     area excluding the line-number display.  */
 	  if (EQ (prop, Qcenter))
 	    return OK_ALIGN_TO (window_box_left_offset (it->w, TEXT_AREA)
-				+ LNUM_ONCE
-				+ window_box_width (it->w, TEXT_AREA) / 2);
+				+ (LNUM_ONCE
+				   + window_box_width (it->w, TEXT_AREA)) / 2);
 	  /* 'left-fringe': left edge of the left fringe.  */
 	  if (EQ (prop, Qleft_fringe))
 	    return OK_ALIGN_TO (WINDOW_HAS_FRINGES_OUTSIDE_MARGINS (it->w)