Fix automatic hscrolling of only the current line

* src/xdisp.c (display_line): When hscrolling only the current
line, increment iterator's first_visible_x and last_visible_x
values to account for the hscroll.  This propagates the hscroll
effect on the iterator geometry all the way down to the
subroutines called by display_line, and avoids scrolling bugs
under large hscroll values.  (Bug#26994)

1 files changed, 39 insertions, 31 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 96bc1a5e032..c0e821a934c 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -20671,8 +20671,11 @@ display_line (struct it *it, int cursor_vpos)
   ptrdiff_t min_pos = ZV + 1, max_pos = 0;
   ptrdiff_t min_bpos UNINIT, max_bpos UNINIT;
   bool pending_handle_line_prefix = false;
+  bool hscroll_this_line = (cursor_vpos >= 0 && it->vpos == cursor_vpos
+                            && hscrolling_current_line_p (it->w));
   int first_visible_x = it->first_visible_x;
   int last_visible_x = it->last_visible_x;
+  int x_incr = 0;
 
   /* We always start displaying at hpos zero even if hscrolled.  */
   eassert (it->hpos == 0 && it->current_x == 0);
@@ -20704,25 +20707,23 @@ display_line (struct it *it, int cursor_vpos)
 
   /* If we are going to display the cursor's line, account for the
      hscroll of that line.  */
-  if (cursor_vpos >= 0 && it->vpos == cursor_vpos
-      && hscrolling_current_line_p (it->w))
-    {
-      int x_incr =
-        window_hscroll_limited (it->w, it->f) * FRAME_COLUMN_WIDTH (it->f);
-
-      first_visible_x += x_incr;
-      last_visible_x  += x_incr;
-    }
+  if (hscroll_this_line)
+    x_incr = window_hscroll_limited (it->w, it->f) * FRAME_COLUMN_WIDTH (it->f);
 
   /* Move over display elements that are not visible because we are
      hscrolled.  This may stop at an x-position < first_visible_x
      if the first glyph is partially visible or if we hit a line end.  */
-  if (it->current_x < first_visible_x)
+  if (it->current_x < it->first_visible_x + x_incr)
     {
       enum move_it_result move_result;
 
       this_line_min_pos = row->start.pos;
-      move_result = move_it_in_display_line_to (it, ZV, first_visible_x,
+      if (hscroll_this_line)
+        {
+          it->first_visible_x += x_incr;
+          it->last_visible_x  += x_incr;
+        }
+      move_result = move_it_in_display_line_to (it, ZV, it->first_visible_x,
                                                 MOVE_TO_POS | MOVE_TO_X);
       /* If we are under a large hscroll, move_it_in_display_line_to
          could hit the end of the line without reaching
@@ -20730,10 +20731,10 @@ display_line (struct it *it, int cursor_vpos)
          especially important on a TTY, where we will call
          extend_face_to_end_of_line, which needs to know how many
          blank glyphs to produce.  */
-      if (it->current_x < first_visible_x
+      if (it->current_x < it->first_visible_x
           && (move_result == MOVE_NEWLINE_OR_CR
               || move_result == MOVE_POS_MATCH_OR_ZV))
-        it->current_x = first_visible_x;
+        it->current_x = it->first_visible_x;
 
       /* Record the smallest positions seen while we moved over
          display elements that are not visible.  This is needed by
@@ -20927,7 +20928,7 @@ display_line (struct it *it, int cursor_vpos)
       if (/* Not a newline.  */
           nglyphs > 0
           /* Glyphs produced fit entirely in the line.  */
-          && it->current_x < last_visible_x)
+          && it->current_x < it->last_visible_x)
         {
           it->hpos += nglyphs;
           row->ascent = max (row->ascent, it->max_ascent);
@@ -20937,13 +20938,13 @@ display_line (struct it *it, int cursor_vpos)
                                   it->max_phys_ascent + it->max_phys_descent);
           row->extra_line_spacing = max (row->extra_line_spacing,
                                          it->max_extra_line_spacing);
-          if (it->current_x - it->pixel_width < first_visible_x
+          if (it->current_x - it->pixel_width < it->first_visible_x
               /* In R2L rows, we arrange in extend_face_to_end_of_line
                  to add a right offset to the line, by a suitable
                  change to the stretch glyph that is the leftmost
                  glyph of the line.  */
               && !row->reversed_p)
-            row->x = x - first_visible_x;
+            row->x = x - it->first_visible_x;
           /* Record the maximum and minimum buffer positions seen so
              far in glyphs that will be displayed by this row.  */
           if (it->bidi_p)
@@ -20968,9 +20969,9 @@ display_line (struct it *it, int cursor_vpos)
               if (/* Lines are continued.  */
                   it->line_wrap != TRUNCATE
                   && (/* Glyph doesn't fit on the line.  */
-                      new_x > last_visible_x
+                      new_x > it->last_visible_x
                       /* Or it fits exactly on a window system frame.  */
-                      || (new_x == last_visible_x
+                      || (new_x == it->last_visible_x
                           && FRAME_WINDOW_P (it->f)
                           && (row->reversed_p
                               ? WINDOW_LEFT_FRINGE_WIDTH (it->w)
@@ -20979,7 +20980,7 @@ display_line (struct it *it, int cursor_vpos)
                   /* End of a continued line.  */
 
                   if (it->hpos == 0
-                      || (new_x == last_visible_x
+                      || (new_x == it->last_visible_x
                           && FRAME_WINDOW_P (it->f)
                           && (row->reversed_p
                               ? WINDOW_LEFT_FRINGE_WIDTH (it->w)
@@ -21122,10 +21123,10 @@ display_line (struct it *it, int cursor_vpos)
                            ? WINDOW_LEFT_FRINGE_WIDTH (it->w)
                            : WINDOW_RIGHT_FRINGE_WIDTH (it->w)) == 0)
                         produce_special_glyphs (it, IT_CONTINUATION);
-                      it->continuation_lines_width += last_visible_x;
+                      it->continuation_lines_width += it->last_visible_x;
                       row->ends_in_middle_of_char_p = true;
                       row->continued_p = true;
-                      glyph->pixel_width = last_visible_x - x;
+                      glyph->pixel_width = it->last_visible_x - x;
                       it->starts_in_middle_of_char_p = true;
                       if (WINDOW_LEFT_MARGIN_WIDTH (it->w) > 0
                           || WINDOW_RIGHT_MARGIN_WIDTH (it->w) > 0)
@@ -21169,7 +21170,7 @@ display_line (struct it *it, int cursor_vpos)
 
                   break;
                 }
-              else if (new_x > first_visible_x)
+              else if (new_x > it->first_visible_x)
                 {
                   /* Increment number of glyphs actually displayed.  */
                   ++it->hpos;
@@ -21180,14 +21181,14 @@ display_line (struct it *it, int cursor_vpos)
                   if (it->bidi_p)
                     RECORD_MAX_MIN_POS (it);
 
-                  if (x < first_visible_x && !row->reversed_p)
+                  if (x < it->first_visible_x && !row->reversed_p)
                     /* Glyph is partially visible, i.e. row starts at
                        negative X position.  Don't do that in R2L
                        rows, where we arrange to add a right offset to
                        the line in extend_face_to_end_of_line, by a
                        suitable change to the stretch glyph that is
                        the leftmost glyph of the line.  */
-                    row->x = x - first_visible_x;
+                    row->x = x - it->first_visible_x;
                   /* When the last glyph of an R2L row only fits
                      partially on the line, we need to set row->x to a
                      negative offset, so that the leftmost glyph is
@@ -21195,12 +21196,12 @@ display_line (struct it *it, int cursor_vpos)
                      going to produce the truncation glyph, this will
                      be taken care of in produce_special_glyphs.  */
                   if (row->reversed_p
-                      && new_x > last_visible_x
+                      && new_x > it->last_visible_x
                       && !(it->line_wrap == TRUNCATE
                            && WINDOW_LEFT_FRINGE_WIDTH (it->w) == 0))
                     {
                       eassert (FRAME_WINDOW_P (it->f));
-                      row->x = last_visible_x - new_x;
+                      row->x = it->last_visible_x - new_x;
                     }
                 }
               else
@@ -21210,7 +21211,7 @@ display_line (struct it *it, int cursor_vpos)
                      move_it_in_display_line at the start of this
                      function, unless the text display area of the
                      window is empty.  */
-                  eassert (first_visible_x <= last_visible_x);
+                  eassert (it->first_visible_x <= it->last_visible_x);
                 }
             }
           /* Even if this display element produced no glyphs at all,
@@ -21279,8 +21280,8 @@ display_line (struct it *it, int cursor_vpos)
                     ? WINDOW_LEFT_FRINGE_WIDTH (it->w)
                     : WINDOW_RIGHT_FRINGE_WIDTH (it->w))
                    || it->what == IT_IMAGE))
-              ? (it->current_x >= last_visible_x)
-              : (it->current_x > last_visible_x)))
+              ? (it->current_x >= it->last_visible_x)
+              : (it->current_x > it->last_visible_x)))
         {
           /* Maybe add truncation glyphs.  */
           if (!FRAME_WINDOW_P (it->f)
@@ -21314,7 +21315,7 @@ display_line (struct it *it, int cursor_vpos)
               /* produce_special_glyphs overwrites the last glyph, so
                  we don't want that if we want to keep that last
                  glyph, which means it's an image.  */
-              if (it->current_x > last_visible_x)
+              if (it->current_x > it->last_visible_x)
                 {
                   it->current_x = x_before;
                   if (!FRAME_WINDOW_P (it->f))
@@ -21375,7 +21376,7 @@ display_line (struct it *it, int cursor_vpos)
 
   /* If line is not empty and hscrolled, maybe insert truncation glyphs
      at the left window margin.  */
-  if (first_visible_x
+  if (it->first_visible_x
       && IT_CHARPOS (*it) != CHARPOS (row->start.pos))
     {
       if (!FRAME_WINDOW_P (it->f)
@@ -21503,6 +21504,13 @@ display_line (struct it *it, int cursor_vpos)
      row to be used.  */
   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.  */
+  if (hscroll_this_line)
+    {
+      it->first_visible_x = first_visible_x;
+      it->last_visible_x  = last_visible_x;
+    }
   SET_TEXT_POS (it->eol_pos, 0, 0);
   ++it->vpos;
   ++it->glyph_row;