Fix use of ':align-to' in 'wrap-prefix'

* src/dispextern.h (struct it): New flag 'align_visually_p'.
* src/xdisp.c (handle_line_prefix): Set the 'align_visually_p'
flag for 'wrap-prefix'.
(produce_stretch_glyph): If 'align_visually_p' flag is set, count
the :align-to offset from the beginning of the screen line, not
from BOL.  (Bug#71605)

* doc/lispref/display.texi (Truncation, Specified Space): Document
the special handling of ':align-to' in 'wrap-prefix'.

3 files changed, 19 insertions, 3 deletions

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index d5c96d13e02..34096196df4 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -192,7 +192,9 @@ never used.)  Its value may be a string or an image (@pxref{Other
 Display Specs}), or a stretch of whitespace such as specified by the
 @code{:width} or @code{:align-to} display properties (@pxref{Specified
 Space}).  The value is interpreted in the same way as a @code{display}
-text property.  @xref{Display Property}.
+text property, with one important difference: the horizontal position
+specified by @code{:align-to} is measured from the visual beginning of
+the screen line.  @xref{Display Property}.
 
 A wrap prefix may also be specified for regions of text, using the
 @code{wrap-prefix} text or overlay property.  This takes precedence
@@ -5354,7 +5356,9 @@ Scrolling}), @var{hpos} is measured from the beginning of the logical
 line, not from the visual beginning of the screen line.  This way,
 alignment produced by @code{:align-to} is consistent with functions
 that count columns, such as @code{current-column} and
-@code{move-to-column} (@pxref{Columns}).
+@code{move-to-column} (@pxref{Columns}).  (There's a single exception
+from this rule: when @code{:align-to} is used to specify whitespace of
+the @code{wrap-prefix} variable or text property, @pxref{Truncation}.)
 @end table
 
   You should use one and only one of the above properties.  You can
diff --git a/src/dispextern.h b/src/dispextern.h
index 85012130689..51dc354d37c 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -2629,6 +2629,11 @@ struct it
      the current row.  */
   bool_bf line_number_produced_p : 1;
 
+  /* If true, the :align-to argument should be counted relative to the
+     beginning of the screen line, not the logical line.  Used by
+     'wrap-prefix'.  */
+  bool_bf align_visually_p : 1;
+
   enum line_wrap_method line_wrap;
 
   /* The ID of the default face to use.  One of DEFAULT_FACE_ID,
diff --git a/src/xdisp.c b/src/xdisp.c
index 0148cd76ada..5987813cd28 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -24494,6 +24494,11 @@ handle_line_prefix (struct it *it)
       prefix = get_line_prefix_it_property (it, Qwrap_prefix);
       if (NILP (prefix))
         prefix = Vwrap_prefix;
+      /* Interpreting :align-to relative to the beginning of the logical
+         line effectively renders this feature unusable, so we make an
+         exception for this use of :align-to.  */
+      if (!NILP (prefix))
+        it->align_visually_p = true;
     }
   else
     {
@@ -31972,7 +31977,9 @@ produce_stretch_glyph (struct it *it)
            && calc_pixel_width_or_height (&tem, it, prop, font, true,
                                           &align_to))
     {
-      int x = it->current_x + it->continuation_lines_width;
+      int x = it->current_x + (it->align_visually_p
+                               ? 0
+                               : it->continuation_lines_width);
       int x0 = x;
       /* Adjust for line numbers, if needed.   */
       if (!NILP (Vdisplay_line_numbers) && it->line_number_produced_p)