Prevent continuation from affecting tab width in/after line prefix

* src/dispextern.h (struct it) <wrap_prefix_width>: New field,
synchronized with current_x when producing glyphs for wrap
prefixes, and subtracted from it->current_x when computing tab
widths.

* src/term.c (produce_glyphs): Set wrap_prefix_width.

* src/xdisp.c (start_display, display_min_width, move_it_to)
(move_it_vertically_backward, move_it_by_lines)
(window_text_pixel_size, display_tab_bar_line)
(display_tool_bar_line, redisplay_internal, redisplay_window)
(try_window_id, insert_left_trunc_glyphs)
(extend_face_to_end_of_line, display_line)
(Fmove_point_visually): Set or clear wrap_prefix_width as
appropriate.
(gui_produce_glyphs): Set or clear it->wrap_prefix_width.  When
computing the base position of a tab character, do not subtract
the continuation line width if a line prefix is the current
iterator method.  Subtract the wrap_prefix_width otherwise, in
order that the width of the tab is computed free of influence
from the wrap prefix.

3 files changed, 74 insertions, 18 deletions

diff --git a/src/dispextern.h b/src/dispextern.h
index 84b9dadc184..5387cb45603 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -2752,6 +2752,16 @@ struct it
      pixel_width with each call to produce_glyphs.  */
   int current_x;
 
+  /* Pixel position within a display line with a wrap prefix.  Updated
+     to reflect current_x in produce_glyphs when producing glyphs from
+     a prefix string and continuation_lines_width > 0, which is to
+     say, from a wrap prefix.
+
+     Such updates are unnecessary where it is impossible for a wrap
+     prefix to be active, e.g. when continuation lines are being
+     produced.  */
+  int wrap_prefix_width;
+
   /* Accumulated width of continuation lines.  If > 0, this means we
      are currently in a continuation line.  This is initially zero and
      incremented/reset by display_line, move_it_to etc.  */
diff --git a/src/term.c b/src/term.c
index 447876d288a..b3793088fac 100644
--- a/src/term.c
+++ b/src/term.c
@@ -1704,7 +1704,13 @@ produce_glyphs (struct it *it)
   /* Advance current_x by the pixel width as a convenience for
      the caller.  */
   if (it->area == TEXT_AREA)
-    it->current_x += it->pixel_width;
+    {
+      it->current_x += it->pixel_width;
+
+      if (it->continuation_lines_width
+          && it->string_from_prefix_prop_p)
+        it->wrap_prefix_width = it->current_x;
+    }
   it->ascent = it->max_ascent = it->phys_ascent = it->max_phys_ascent = 0;
   it->descent = it->max_descent = it->phys_descent = it->max_phys_descent = 1;
 #endif
diff --git a/src/xdisp.c b/src/xdisp.c
index 066217a2f0f..4ff689b2df7 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3821,7 +3821,7 @@ start_display (struct it *it, struct window *w, struct text_pos pos)
 
           it->current_y = first_y;
           it->vpos = 0;
-          it->current_x = it->hpos = 0;
+          it->current_x = it->hpos = it->wrap_prefix_width = 0;
         }
     }
 }
@@ -5532,7 +5532,13 @@ display_min_width (struct it *it, ptrdiff_t bufpos,
           it->object = list3 (Qspace, QCwidth, w);
           produce_stretch_glyph (it);
           if (it->area == TEXT_AREA)
-            it->current_x += it->pixel_width;
+            {
+              it->current_x += it->pixel_width;
+
+              if (it->continuation_lines_width
+                  && it->string_from_prefix_prop_p)
+                it->wrap_prefix_width = it->current_x;
+            }
           it->min_width_property = Qnil;
         }
     }
@@ -10797,6 +10803,7 @@ move_it_to (struct it *it, ptrdiff_t to_charpos, int to_x, int to_y, int to_vpos
 
       /* Reset/increment for the next run.  */
       it->current_x = line_start_x;
+      it->wrap_prefix_width = 0;
       line_start_x = 0;
       it->hpos = 0;
       it->line_number_produced_p = false;
@@ -10827,6 +10834,7 @@ move_it_to (struct it *it, ptrdiff_t to_charpos, int to_x, int to_y, int to_vpos
     {
       it->continuation_lines_width += it->current_x;
       it->current_x = it->hpos = it->max_ascent = it->max_descent = 0;
+      it->wrap_prefix_width = 0;
       it->current_y += it->max_ascent + it->max_descent;
       ++it->vpos;
       last_height = it->max_ascent + it->max_descent;
@@ -10886,6 +10894,7 @@ move_it_vertically_backward (struct it *it, int dy)
   reseat_1 (it, it->current.pos, true);
 
   /* We are now surely at a line start.  */
+  it->wrap_prefix_width = 0;
   it->current_x = it->hpos = 0; /* FIXME: this is incorrect when bidi
                                    reordering is in effect.  */
   it->continuation_lines_width = 0;
@@ -11164,7 +11173,7 @@ move_it_by_lines (struct it *it, ptrdiff_t dvpos)
           dvpos--;
         }
 
-      it->current_x = it->hpos = 0;
+      it->current_x = it->hpos = it->wrap_prefix_width = 0;
 
       /* Above call may have moved too far if continuation lines
          are involved.  Scan forward and see if it did.  */
@@ -11173,7 +11182,7 @@ move_it_by_lines (struct it *it, ptrdiff_t dvpos)
       move_it_to (&it2, start_charpos, -1, -1, -1, MOVE_TO_POS);
       it->vpos -= it2.vpos;
       it->current_y -= it2.current_y;
-      it->current_x = it->hpos = 0;
+      it->current_x = it->hpos = it->wrap_prefix_width = 0;
 
       /* If we moved too far back, move IT some lines forward.  */
       if (it2.vpos > -dvpos)
@@ -11452,7 +11461,7 @@ window_text_pixel_size (Lisp_Object window, Lisp_Object from, Lisp_Object to,
          IT.current_x will be incorrectly set to zero at some arbitrary
          non-zero X coordinate.  */
       move_it_by_lines (&it, 0);
-      it.current_x = it.hpos = 0;
+      it.current_x = it.hpos = it.wrap_prefix_width = 0;
       if (IT_CHARPOS (it) != start)
         {
           void *it1data = NULL;
@@ -11505,7 +11514,7 @@ window_text_pixel_size (Lisp_Object window, Lisp_Object from, Lisp_Object to,
   /* If FROM is on a newline, pretend that we start at the beginning
      of the next line, because the newline takes no place on display.  */
   if (FETCH_BYTE (start) == '\n')
-    it.current_x = 0;
+    it.current_x = 0, it.wrap_prefix_width = 0;
   if (!NILP (x_limit))
     {
       it.last_visible_x = max_x;
@@ -14417,7 +14426,7 @@ display_tab_bar_line (struct it *it, int height)
   row->truncated_on_left_p = false;
   row->truncated_on_right_p = false;
 
-  it->current_x = it->hpos = 0;
+  it->current_x = it->hpos = it->wrap_prefix_width = 0;
   it->current_y += row->height;
   ++it->vpos;
   ++it->glyph_row;
@@ -15441,7 +15450,7 @@ display_tool_bar_line (struct it *it, int height)
   row->truncated_on_left_p = false;
   row->truncated_on_right_p = false;
 
-  it->current_x = it->hpos = 0;
+  it->current_x = it->hpos = it->wrap_prefix_width = 0;
   it->current_y += row->height;
   ++it->vpos;
   ++it->glyph_row;
@@ -17141,6 +17150,7 @@ redisplay_internal (void)
                          NULL, DEFAULT_FACE_ID);
           it.current_x = this_line_start_x;
           it.current_y = this_line_y;
+          it.wrap_prefix_width = 0;
           it.vpos = this_line_vpos;
 
           if (current_buffer->long_line_optimizations_p
@@ -20587,7 +20597,7 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
       it.current_y = 0;
     }
 
-  it.current_x = it.hpos = 0;
+  it.current_x = it.wrap_prefix_width = it.hpos = 0;
 
   /* Set the window start position here explicitly, to avoid an
      infinite loop in case the functions in window-scroll-functions
@@ -22555,7 +22565,7 @@ try_window_id (struct window *w)
       /* We may start in a continuation line.  If so, we have to
          get the right continuation_lines_width and current_x.  */
       it.continuation_lines_width = last_row->continuation_lines_width;
-      it.hpos = it.current_x = 0;
+      it.hpos = it.current_x = it.wrap_prefix_width = 0;
 
       /* Display the rest of the lines at the window end.  */
       it.glyph_row = MATRIX_ROW (desired_matrix, it.vpos);
@@ -23160,6 +23170,7 @@ insert_left_trunc_glyphs (struct it *it)
   /* Get the truncation glyphs.  */
   truncate_it = *it;
   truncate_it.current_x = 0;
+  truncate_it.wrap_prefix_width = 0;
   truncate_it.face_id = DEFAULT_FACE_ID;
   truncate_it.glyph_row = &scratch_glyph_row;
   truncate_it.area = TEXT_AREA;
@@ -23922,6 +23933,10 @@ extend_face_to_end_of_line (struct it *it)
           for (it->current_x = 0; g < e; g++)
             it->current_x += g->pixel_width;
 
+          if (it->continuation_lines_width
+              && it->string_from_prefix_prop_p)
+            it->wrap_prefix_width = it->current_x;
+
           it->area = LEFT_MARGIN_AREA;
           it->face_id = default_face->id;
           while (it->glyph_row->used[LEFT_MARGIN_AREA]
@@ -25064,7 +25079,10 @@ display_line (struct it *it, int cursor_vpos)
       if (it->current_x < it->first_visible_x
           && (move_result == MOVE_NEWLINE_OR_CR
               || move_result == MOVE_POS_MATCH_OR_ZV))
-        it->current_x = it->first_visible_x;
+        {
+          it->current_x = it->first_visible_x;
+          it->wrap_prefix_width = 0;
+        }
 
       /* In case move_it_in_display_line_to above "produced" the line
          number.  */
@@ -25921,7 +25939,7 @@ display_line (struct it *it, int cursor_vpos)
      HPOS) = (0 0).  Vertical positions are incremented.  As a
      convenience for the caller, IT->glyph_row is set to the next
      row to be used.  */
-  it->current_x = it->hpos = 0;
+  it->wrap_prefix_width = it->current_x = it->hpos = 0;
   it->current_y += row->height;
   /* Restore the first and last visible X if we adjusted them for
      current-line hscrolling.  */
@@ -26400,7 +26418,7 @@ Value is the new character position of point.  */)
     {
       struct text_pos pt;
       struct it it;
-      int pt_x, target_x, pixel_width, pt_vpos;
+      int pt_x, pt_wrap_prefix_x, target_x, pixel_width, pt_vpos;
       bool at_eol_p;
       bool overshoot_expected = false;
       bool target_is_eol_p = false;
@@ -26432,6 +26450,7 @@ Value is the new character position of point.  */)
     reseat:
       reseat_at_previous_visible_line_start (&it);
       it.current_x = it.hpos = it.current_y = it.vpos = 0;
+      it.wrap_prefix_width = 0;
       if (IT_CHARPOS (it) != PT)
         {
           move_it_to (&it, overshoot_expected ? PT - 1 : PT,
@@ -26450,6 +26469,7 @@ Value is the new character position of point.  */)
             move_it_in_display_line (&it, PT, -1, MOVE_TO_POS);
         }
       pt_x = it.current_x;
+      pt_wrap_prefix_x = it.wrap_prefix_width;
       pt_vpos = it.vpos;
       if (dir > 0 || overshoot_expected)
         {
@@ -26464,10 +26484,11 @@ Value is the new character position of point.  */)
           it.glyph_row = NULL;
           PRODUCE_GLYPHS (&it); /* compute it.pixel_width */
           it.glyph_row = row;
-          /* PRODUCE_GLYPHS advances it.current_x, so we must restore
-             it, lest it will become out of sync with it's buffer
+          /* PRODUCE_GLYPHS advances it.current_x, so it must be
+             restored, lest it become out of sync with its buffer
              position.  */
           it.current_x = pt_x;
+          it.wrap_prefix_width = pt_wrap_prefix_x;
         }
       else
         at_eol_p = ITERATOR_AT_END_OF_LINE_P (&it);
@@ -26512,6 +26533,7 @@ Value is the new character position of point.  */)
                 it.last_visible_x = DISP_INFINITY;
               reseat_at_previous_visible_line_start (&it);
               it.current_x = it.current_y = it.hpos = 0;
+              it.wrap_prefix_width = 0;
               if (pt_vpos != 0)
                 move_it_by_lines (&it, pt_vpos);
             }
@@ -32659,7 +32681,19 @@ gui_produce_glyphs (struct it *it)
           if (font->space_width > 0)
             {
               int tab_width = it->tab_width * font->space_width;
-              int x = it->current_x + it->continuation_lines_width;
+              /* wrap-prefix strings are prepended to continuation
+                 lines, so the width of tab characters inside should
+                 be computed from the start of this screen line rather
+                 than as a product of the total width of the physical
+                 line being wrapped.  */
+              int x = it->current_x + (it->string_from_prefix_prop_p
+                                       /* Subtract the width of the
+                                          prefix from it->current_x if
+                                          it exists.  */
+                                       ? 0 : (it->continuation_lines_width
+                                              ? (it->continuation_lines_width
+                                                 - it->wrap_prefix_width)
+                                              : 0));
               int x0 = x;
               /* Adjust for line numbers, if needed.   */
               if (!NILP (Vdisplay_line_numbers) && it->line_number_produced_p)
@@ -33130,7 +33164,13 @@ gui_produce_glyphs (struct it *it)
      because this isn't true for images with `:ascent 100'.  */
   eassert (it->ascent >= 0 && it->descent >= 0);
   if (it->area == TEXT_AREA)
-    it->current_x += it->pixel_width;
+    {
+      it->current_x += it->pixel_width;
+
+      if (it->continuation_lines_width
+          && it->string_from_prefix_prop_p)
+        it->wrap_prefix_width = it->current_x;
+    }
 
   if (extra_line_spacing > 0)
     {