Fix vertical cursor motion across tall text or small images

'line-move-partial' should in general leave it to the display
engine to scroll or recenter the window due to vertical motion of
the cursor.  The only purpose of this function is to produce
vscroll suitable for scrolling across large (relatively to the
window's height) images, where moving by display lines is not
appropriate.

* src/xdisp.c (Fdisplay__line_is_continued_p): New primitive.

* lisp/simple.el (line-move-partial): Call
'display--line-is-continued-p' to decide whether to leave it to
redisplay to scroll the window as appropriate.  (Bug#48170)

1 files changed, 42 insertions, 0 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 23b4ba5c39c..4841a0af6f3 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -10838,6 +10838,47 @@ include the height of both, if present, in the return value.  */)
 
   return Fcons (make_fixnum (x - start_x), make_fixnum (y));
 }
+
+DEFUN ("display--line-is-continued-p", Fdisplay__line_is_continued_p,
+       Sdisplay__line_is_continued_p, 0, 0, 0,
+       doc: /* Return non-nil if the current screen line is continued on display.  */)
+  (void)
+{
+  struct buffer *oldb = current_buffer;
+  struct window *w = XWINDOW (selected_window);
+  enum move_it_result rc = MOVE_POS_MATCH_OR_ZV;
+
+  set_buffer_internal_1 (XBUFFER (w->contents));
+
+  if (PT < ZV)
+    {
+      struct text_pos startpos;
+      struct it it;
+      void *itdata;
+      /* Use a marker, since vertical-motion enters redisplay, which can
+         trigger fontifications, which in turn could modify buffer text.  */
+      Lisp_Object opoint = Fpoint_marker ();
+
+      /* Make sure to start from the beginning of the current screen
+         line, so that move_it_in_display_line_to counts pixels correctly.  */
+      Fvertical_motion (make_fixnum (0), selected_window, Qnil);
+      SET_TEXT_POS (startpos, PT, PT_BYTE);
+      itdata = bidi_shelve_cache ();
+      start_display (&it, w, startpos);
+      /* If lines are truncated, no line is continued.  */
+      if (it.line_wrap != TRUNCATE)
+        {
+          it.glyph_row = NULL;
+          rc = move_it_in_display_line_to (&it, ZV, -1, MOVE_TO_POS);
+        }
+      SET_PT_BOTH (marker_position (opoint), marker_byte_position (opoint));
+      bidi_unshelve_cache (itdata, false);
+    }
+  set_buffer_internal_1 (oldb);
+
+  return rc == MOVE_LINE_CONTINUED ? Qt : Qnil;
+}
+
 
 /***********************************************************************
                                Messages
@@ -34739,6 +34780,7 @@ be let-bound around code that needs to disable messages temporarily. */);
   defsubr (&Swindow_text_pixel_size);
   defsubr (&Smove_point_visually);
   defsubr (&Sbidi_find_overridden_directionality);
+  defsubr (&Sdisplay__line_is_continued_p);
 
   DEFSYM (Qmenu_bar_update_hook, "menu-bar-update-hook");
   DEFSYM (Qoverriding_terminal_local_map, "overriding-terminal-local-map");