Allow ensuring that a window's starting point is never invisible

* src/xdisp.c (syms_of_xdisp) <make-window-start-visible>: New
buffer-local variable.
(redisplay_window): If 'make-window-start-visible' is non-nil,
don't accept window-start position that is in invisible text or is
covered by a "replacing" 'display' property.  (Bug#14582)

1 files changed, 47 insertions, 5 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 381df490703..20b0d97b975 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -19036,8 +19036,9 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
  force_start:
 
   /* Handle case where place to start displaying has been specified,
-     unless the specified location is outside the accessible range.  */
-  if (w->force_start)
+     unless the specified location is outside the accessible range, or
+     the buffer wants the window-start point to be always visible.  */
+  if (w->force_start && !make_window_start_visible)
     {
       /* We set this later on if we have to adjust point.  */
       int new_vpos = -1;
@@ -19227,6 +19228,8 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
       goto done;
     }
 
+  Lisp_Object iprop, dspec;
+  struct text_pos ignored;
   /* Handle case where text has not changed, only point, and it has
      not moved off the frame, and we are not retrying after hscroll.
      (current_matrix_up_to_date_p is true when retrying.)  */
@@ -19248,10 +19251,28 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
         }
     }
   /* If current starting point was originally the beginning of a line
-     but no longer is, find a new starting point.  */
+     but no longer is, or if the starting point is invisible but the
+     buffer wants it always visible, find a new starting point.  */
   else if (w->start_at_line_beg
-           && !(CHARPOS (startp) <= BEGV
-                || FETCH_BYTE (BYTEPOS (startp) - 1) == '\n'))
+           && (!(CHARPOS (startp) <= BEGV
+                 || FETCH_BYTE (BYTEPOS (startp) - 1) == '\n')
+               || (make_window_start_visible
+                   /* Is window-start in invisible text?  */
+                   && ((CHARPOS (startp) > BEGV
+                        && ((iprop =
+                             Fget_char_property
+                             (make_fixnum (CHARPOS (startp) - 1), Qinvisible,
+                              window)),
+                            TEXT_PROP_MEANS_INVISIBLE (iprop) != 0))
+                       /* Is window-start covered by a replacing
+                          'display' property?  */
+                       || (!NILP (dspec =
+                                  Fget_char_property
+                                  (make_fixnum (CHARPOS (startp)), Qdisplay,
+                                   window))
+                           && handle_display_spec (NULL, dspec, Qnil, Qnil,
+                                                   &ignored, CHARPOS (startp),
+                                                   FRAME_WINDOW_P (f)) > 0)))))
     {
 #ifdef GLYPH_DEBUG
       debug_method_add (w, "recenter 1");
@@ -19327,6 +19348,21 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
           goto force_start;
         }
 
+      /* Don't use the same window-start if it is covered by a
+         replacing 'display' property and the buffer requested the
+         window-start to be always visible.  */
+      if (make_window_start_visible
+          && !NILP (dspec = Fget_char_property (make_fixnum (CHARPOS (startp)),
+                                                Qdisplay, window))
+          && handle_display_spec (NULL, dspec, Qnil, Qnil, &ignored,
+                                  CHARPOS (startp), FRAME_WINDOW_P (f)) > 0)
+        {
+#ifdef GLYPH_DEBUG
+          debug_method_add (w, "recenter 2");
+#endif
+          goto recenter;
+        }
+
 #ifdef GLYPH_DEBUG
       debug_method_add (w, "same window start");
 #endif
@@ -35973,6 +36009,12 @@ window, nil if it's okay to leave the cursor partially-visible.  */);
   Vmake_cursor_line_fully_visible = Qt;
   DEFSYM (Qmake_cursor_line_fully_visible, "make-cursor-line-fully-visible");
 
+  DEFVAR_BOOL ("make-window-start-visible", make_window_start_visible,
+    doc: /* Whether to ensure `window-start' position is never invisible.  */);
+  make_window_start_visible = false;
+  DEFSYM (Qmake_window_start_visible, "make-window-start-visible");
+  Fmake_variable_buffer_local (Qmake_window_start_visible);
+
   DEFSYM (Qclose_tab, "close-tab");
   DEFVAR_LISP ("tab-bar-border", Vtab_bar_border,
     doc: /* Border below tab-bar in pixels.