From 239fe08b7667e397913842fe4a7fc5784dd5ea27 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs@gmail.com>
Date: Sat, 21 Jan 2017 13:24:47 -0500
Subject: [PATCH v4 4/5] Fix scrolling with partial lines

* src/xdisp.c (partial_line_height): New function.
(try_scrolling):
* src/window.c (window_scroll_pixel_based): Use it for calculating the
pixel scroll margin correctly in a window with partial lines.
---
 src/dispextern.h |  1 +
 src/window.c     |  2 +-
 src/xdisp.c      | 29 ++++++++++++++++++++++++++++-
 3 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/src/dispextern.h b/src/dispextern.h
index 51222e6..eb71a82 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -3263,6 +3263,7 @@ void move_it_past_eol (struct it *);
 void move_it_in_display_line (struct it *it,
 			      ptrdiff_t to_charpos, int to_x,
 			      enum move_operation_enum op);
+int partial_line_height (struct it *it_origin);
 bool in_display_vector_p (struct it *);
 int frame_mode_line_height (struct frame *);
 extern bool redisplaying_p;
diff --git a/src/window.c b/src/window.c
index 7d86d9d..9feeede 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5149,7 +5149,7 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
 	 in the scroll margin at the bottom.  */
       move_it_to (&it, PT, -1,
 		  (it.last_visible_y - WINDOW_HEADER_LINE_HEIGHT (w)
-		   - this_scroll_margin - 1),
+                   - partial_line_height (&it) - this_scroll_margin - 1),
 		  -1,
 		  MOVE_TO_POS | MOVE_TO_Y);
 
diff --git a/src/xdisp.c b/src/xdisp.c
index 134ef6c..0e329df 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -9859,6 +9859,32 @@ move_it_by_lines (struct it *it, ptrdiff_t dvpos)
     }
 }
 
+int
+partial_line_height (struct it *it_origin)
+{
+  int partial_height;
+  void *it_data = NULL;
+  struct it it;
+  SAVE_IT (it, *it_origin, it_data);
+  move_it_to (&it, ZV, -1, it.last_visible_y, -1,
+              MOVE_TO_POS | MOVE_TO_Y);
+  if (it.what == IT_EOB)
+    {
+      int vis_height = it.last_visible_y - it.current_y;
+      int height = it.ascent + it.descent;
+      partial_height = (vis_height < height) ? vis_height : 0;
+    }
+  else
+    {
+      int last_line_y = it.current_y;
+      move_it_by_lines (&it, 1);
+      partial_height = (it.current_y > it.last_visible_y)
+        ? it.last_visible_y - last_line_y : 0;
+    }
+  RESTORE_IT (&it, &it, it_data);
+  return partial_height;
+}
+
 /* Return true if IT points into the middle of a display vector.  */
 
 bool
@@ -15368,7 +15394,8 @@ try_scrolling (Lisp_Object window, bool just_this_one_p,
       /* Compute the pixel ypos of the scroll margin, then move IT to
 	 either that ypos or PT, whichever comes first.  */
       start_display (&it, w, startp);
-      scroll_margin_y = it.last_visible_y - this_scroll_margin
+      scroll_margin_y = it.last_visible_y - partial_line_height (&it)
+        - this_scroll_margin
 	- frame_line_height * extra_scroll_margin_lines;
       move_it_to (&it, PT, -1, scroll_margin_y - 1, -1,
 		  (MOVE_TO_POS | MOVE_TO_Y));
-- 
2.9.3