Support lower bound on hscrolling when only current line scrolls

* doc/emacs/display.texi (Horizontal Scrolling): Document the new
mode of auto-hscrolling only the current line.

* src/xdisp.c (init_iterator): When hscrolling only the
current line, apply the	window's min_hscroll here, so that
non-current lines will be hscrolled by that minimum.
Suggested by Stephen Berman <stephen.berman@gmx.net>.
(hscroll_window_tree): Account for window's min_hscroll when
deciding whether to recompute the hscroll.
(display_line): Subtract window's min_hscroll from x_incr, as that
was already accounted for in init_iterator.  (Bug#27008)

1 files changed, 20 insertions, 4 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index f4461c1627a..eaa701e9cf1 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -2890,8 +2890,19 @@ init_iterator (struct it *it, struct window *w,
     }
   else
     {
+      /* When hscrolling only the current line, don't apply the
+         hscroll here, it will be applied by display_line when it gets
+         to laying out the line showing point.  However, if the
+         window's min_hscroll is positive, the user specified a lower
+         bound for automatic hscrolling, so they expect the
+         non-current lines to obey that hscroll amount.  */
       if (hscrolling_current_line_p (w))
-        it->first_visible_x = 0;
+        {
+          if (w->min_hscroll > 0)
+            it->first_visible_x = w->min_hscroll * FRAME_COLUMN_WIDTH (it->f);
+          else
+            it->first_visible_x = 0;
+        }
       else
         it->first_visible_x =
           window_hscroll_limited (w, it->f) * FRAME_COLUMN_WIDTH (it->f);
@@ -13099,7 +13110,9 @@ hscroll_window_tree (Lisp_Object window)
                      that doesn't need to be hscrolled.  If we omit
                      this condition, the line from which we move will
                      remain hscrolled.  */
-                  || (hscl && w->hscroll && !cursor_row->truncated_on_left_p)))
+                  || (hscl
+                      && w->hscroll != w->min_hscroll
+                      && !cursor_row->truncated_on_left_p)))
             {
               struct it it;
               ptrdiff_t hscroll;
@@ -20717,9 +20730,12 @@ display_line (struct it *it, int cursor_vpos)
   recenter_overlay_lists (current_buffer, IT_CHARPOS (*it));
 
   /* If we are going to display the cursor's line, account for the
-     hscroll of that line.  */
+     hscroll of that line.  We subtract the window's min_hscroll,
+     because that was already accounted for in init_iterator.  */
   if (hscroll_this_line)
-    x_incr = window_hscroll_limited (it->w, it->f) * FRAME_COLUMN_WIDTH (it->f);
+    x_incr =
+      (window_hscroll_limited (it->w, it->f) - it->w->min_hscroll)
+      * 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