src/xdisp.c (hscroll_window_tree): Support hscroll in right-to-left lines.

2 files changed, 32 insertions, 6 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index edfecaa0930..3f93987c0f1 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,8 @@
 2011-10-08  Eli Zaretskii  <eliz@gnu.org>
 
+        * xdisp.c (hscroll_window_tree): Support hscroll in right-to-left
+        lines.
+
         * dispnew.c (buffer_posn_from_coords): Account for a possible
         presence of header-line.  (Bug#4426)
 
diff --git a/src/xdisp.c b/src/xdisp.c
index a072ff2c171..2fff6d9518c 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -12065,6 +12065,7 @@ hscroll_window_tree (Lisp_Object window)
             = (desired_cursor_row->enabled_p
                ? desired_cursor_row
                : current_cursor_row);
+          int row_r2l_p = cursor_row->reversed_p;
 
           text_area_width = window_box_width (w, TEXT_AREA);
 
@@ -12072,11 +12073,31 @@ hscroll_window_tree (Lisp_Object window)
           h_margin = hscroll_margin * WINDOW_FRAME_COLUMN_WIDTH (w);
 
           if (!NILP (Fbuffer_local_value (Qauto_hscroll_mode, w->buffer))
-              && ((XFASTINT (w->hscroll)
-                   && w->cursor.x <= h_margin)
-                  || (cursor_row->enabled_p
-                      && cursor_row->truncated_on_right_p
-                      && (w->cursor.x >= text_area_width - h_margin))))
+              /* For left-to-right rows, hscroll when cursor is either
+                 (i) inside the right hscroll margin, or (ii) if it is
+                 inside the left margin and the window is already
+                 hscrolled. */
+              && ((!row_r2l_p
+                   && ((XFASTINT (w->hscroll)
+                        && w->cursor.x <= h_margin)
+                       || (cursor_row->enabled_p
+                           && cursor_row->truncated_on_right_p
+                           && (w->cursor.x >= text_area_width - h_margin))))
+                  /* For right-to-left rows, the logic is similar,
+                     except that rules for scrolling to left and right
+                     are reversed.  E.g., if cursor.x <= h_margin, we
+                     need to hscroll "to the right" unconditionally,
+                     and that will scroll the screen to the left so as
+                     to reveal the next portion of the row.  */
+                  || (row_r2l_p
+                      && ((cursor_row->enabled_p
+                           /* FIXME: It is confusing to set the
+                              truncated_on_right_p flag when R2L rows
+                              are actually truncated on the left. */
+                           && cursor_row->truncated_on_right_p
+                           && w->cursor.x <= h_margin)
+                          || (XFASTINT (w->hscroll)
+                              && (w->cursor.x >= text_area_width - h_margin))))))
             {
               struct it it;
               int hscroll;
@@ -12111,7 +12132,9 @@ hscroll_window_tree (Lisp_Object window)
                                       ? (text_area_width - 4 * FRAME_COLUMN_WIDTH (it.f))
                                       : (text_area_width / 2))))
                           / FRAME_COLUMN_WIDTH (it.f);
-              else if (w->cursor.x >= text_area_width - h_margin)
+              else if ((!row_r2l_p
+                        && w->cursor.x >= text_area_width - h_margin)
+                       || (row_r2l_p && w->cursor.x <= h_margin))
                 {
                   if (hscroll_relative_p)
                     wanted_x = text_area_width * (1 - hscroll_step_rel)