From 03669ccbad9fcaf6c29003d23f3e8bdecaab21b1 Mon Sep 17 00:00:00 2001
From: Eli Zaretskii <eliz@gnu.org>
Date: Sat, 8 Oct 2011 14:15:08 +0200
Subject: [PATCH] src/xdisp.c (hscroll_window_tree): Support hscroll in
 right-to-left lines.

---
 src/ChangeLog |  3 +++
 src/xdisp.c   | 35 +++++++++++++++++++++++++++++------
 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)
-- 
2.11.4.GIT