Allow 'make-cursor-line-fully-visible' name a function

* src/xdisp.c (cursor_row_fully_visible_p): Handle the case of
make-cursor-line-fully-visible being a function.  Accept a 3rd
argument; if non-zero, assume the caller already tested the
conditions for the cursor being fully-visible, and don't
recheck them.  All callers changed.
(try_cursor_movement, try_window_id): Call
cursor_row_fully_visible_p instead of testing the value of
make-cursor-line-fully-visible directly.
(syms_of_xdisp) <make-cursor-line-fully-visible>: Update the
doc string.  Define a symbol Qmake_cursor_line_fully_visible.
(Bug#32848)

* lisp/cus-start.el (standard): Update the Custom form.

* etc/NEWS: Mention the change in possible values of
'make-cursor-line-fully-visible'.

1 files changed, 52 insertions, 17 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 93cd54a3240..d61d421f08a 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -842,7 +842,7 @@ static Lisp_Object redisplay_window_1 (Lisp_Object);
 static bool set_cursor_from_row (struct window *, struct glyph_row *,
                                  struct glyph_matrix *, ptrdiff_t, ptrdiff_t,
                                  int, int);
-static bool cursor_row_fully_visible_p (struct window *, bool, bool);
+static bool cursor_row_fully_visible_p (struct window *, bool, bool, bool);
 static bool update_menu_bar (struct frame *, bool, bool);
 static bool try_window_reusing_current_matrix (struct window *);
 static int try_window_id (struct window *);
@@ -14346,7 +14346,7 @@ redisplay_internal (void)
               eassert (this_line_vpos == it.vpos);
               eassert (this_line_y == it.current_y);
               set_cursor_from_row (w, row, w->current_matrix, 0, 0, 0, 0);
-              if (cursor_row_fully_visible_p (w, false, true))
+              if (cursor_row_fully_visible_p (w, false, true, false))
                 {
 #ifdef GLYPH_DEBUG
                   *w->desired_matrix->method = 0;
@@ -15628,19 +15628,46 @@ run_window_scroll_functions (Lisp_Object window, struct text_pos startp)
    window's current glyph matrix; otherwise use the desired glyph
    matrix.
 
+   If JUST_TEST_USER_PREFERENCE_P, just test what the value of
+   make-cursor-row-fully-visible requires, don't test the actual
+   cursor position.  The assumption is that in that case the caller
+   performs the necessary testing of the cursor position.
+
    A value of false means the caller should do scrolling
    as if point had gone off the screen.  */
 
 static bool
 cursor_row_fully_visible_p (struct window *w, bool force_p,
-                            bool current_matrix_p)
+                            bool current_matrix_p,
+                            bool just_test_user_preference_p)
 {
   struct glyph_matrix *matrix;
   struct glyph_row *row;
   int window_height;
+  Lisp_Object mclfv_p =
+    buffer_local_value (Qmake_cursor_line_fully_visible, w->contents);
 
-  if (!make_cursor_line_fully_visible_p)
+  /* If no local binding, use the global value.  */
+  if (EQ (mclfv_p, Qunbound))
+    mclfv_p = Vmake_cursor_line_fully_visible;
+  /* Follow mode sets the variable to a Lisp function in buffers that
+     are under Follow mode.  */
+  if (FUNCTIONP (mclfv_p))
+    {
+      Lisp_Object window;
+      XSETWINDOW (window, w);
+      /* Implementation note: if the function we call here signals an
+         error, we will NOT scroll when the cursor is partially-visible.  */
+      Lisp_Object val = safe_call1 (mclfv_p, window);
+      if (NILP (val))
+        return true;
+      else if (just_test_user_preference_p)
+        return false;
+    }
+  else if (NILP (mclfv_p))
     return true;
+  else if (just_test_user_preference_p)
+    return false;
 
   /* It's not always possible to find the cursor, e.g, when a window
      is full of overlay strings.  Don't do anything in that case.  */
@@ -16002,7 +16029,7 @@ try_scrolling (Lisp_Object window, bool just_this_one_p,
       /* If cursor ends up on a partially visible line,
          treat that as being off the bottom of the screen.  */
       if (! cursor_row_fully_visible_p (w, extra_scroll_margin_lines <= 1,
-                                        false)
+                                        false, false)
           /* It's possible that the cursor is on the first line of the
              buffer, which is partially obscured due to a vscroll
              (Bug#7537).  In that case, avoid looping forever. */
@@ -16367,7 +16394,7 @@ try_cursor_movement (Lisp_Object window, struct text_pos startp,
               /* Make sure this isn't a header line by any chance, since
                  then MATRIX_ROW_PARTIALLY_VISIBLE_P might yield true.  */
               && !row->mode_line_p
-              && make_cursor_line_fully_visible_p)
+              && !cursor_row_fully_visible_p (w, true, true, true))
             {
               if (PT == MATRIX_ROW_END_CHARPOS (row)
                   && !row->ends_at_zv_p
@@ -16385,7 +16412,7 @@ try_cursor_movement (Lisp_Object window, struct text_pos startp,
               else
                 {
                   set_cursor_from_row (w, row, w->current_matrix, 0, 0, 0, 0);
-                  if (!cursor_row_fully_visible_p (w, false, true))
+                  if (!cursor_row_fully_visible_p (w, false, true, false))
                     rc = CURSOR_MOVEMENT_MUST_SCROLL;
                   else
                     rc = CURSOR_MOVEMENT_SUCCESS;
@@ -16964,7 +16991,7 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
             new_vpos = window_box_height (w) / 2;
         }
 
-      if (!cursor_row_fully_visible_p (w, false, false))
+      if (!cursor_row_fully_visible_p (w, false, false, false))
         {
           /* Point does appear, but on a line partly visible at end of window.
              Move it back to a fully-visible line.  */
@@ -17059,7 +17086,8 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
                 goto need_larger_matrices;
             }
         }
-      if (w->cursor.vpos < 0 || !cursor_row_fully_visible_p (w, false, false))
+      if (w->cursor.vpos < 0
+          || !cursor_row_fully_visible_p (w, false, false, false))
         {
           clear_glyph_matrix (w->desired_matrix);
           goto try_to_scroll;
@@ -17206,7 +17234,7 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
             /* Forget any recorded base line for line number display.  */
             w->base_line_number = 0;
 
-          if (!cursor_row_fully_visible_p (w, true, false))
+          if (!cursor_row_fully_visible_p (w, true, false, false))
             {
               clear_glyph_matrix (w->desired_matrix);
               last_line_misfit = true;
@@ -17502,7 +17530,7 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
       set_cursor_from_row (w, row, matrix, 0, 0, 0, 0);
     }
 
-  if (!cursor_row_fully_visible_p (w, false, false))
+  if (!cursor_row_fully_visible_p (w, false, false, false))
     {
       /* If vscroll is enabled, disable it and try again.  */
       if (w->vscroll)
@@ -19068,9 +19096,10 @@ try_window_id (struct window *w)
          && CHARPOS (start) > BEGV)
         /* Old redisplay didn't take scroll margin into account at the bottom,
            but then global-hl-line-mode doesn't scroll.  KFS 2004-06-14 */
-        || (w->cursor.y + (make_cursor_line_fully_visible_p
-                           ? cursor_height + this_scroll_margin
-                           : 1)) > it.last_visible_y)
+        || (w->cursor.y
+            + (cursor_row_fully_visible_p (w, false, true, true)
+               ? 1
+               : cursor_height + this_scroll_margin)) > it.last_visible_y)
       {
         w->cursor.vpos = -1;
         clear_glyph_matrix (w->desired_matrix);
@@ -32903,9 +32932,15 @@ automatically; to decrease the tool-bar height, use \\[recenter].  */);
     doc: /* Non-nil means raise tool-bar buttons when the mouse moves over them.  */);
   auto_raise_tool_bar_buttons_p = true;
 
-  DEFVAR_BOOL ("make-cursor-line-fully-visible", make_cursor_line_fully_visible_p,
-    doc: /* Non-nil means to scroll (recenter) cursor line if it is not fully visible.  */);
-  make_cursor_line_fully_visible_p = true;
+  DEFVAR_LISP ("make-cursor-line-fully-visible", Vmake_cursor_line_fully_visible,
+    doc: /* Whether to scroll the window if the cursor line is not fully visible.
+If the value is non-nil, Emacs scrolls or recenters the window to make
+the cursor line fully visible.  The value could also be a function, which
+is called with a single argument, the window to be scrolled, and should
+return non-nil if the partially-visible cursor requires scrolling the
+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_LISP ("tool-bar-border", Vtool_bar_border,
     doc: /* Border below tool-bar in pixels.