Count just enough lines for half page scrolling

src/drawscreen.c

@@ -1781,7 +1781,7 @@ win_update(win_T *wp)
 	    if (j < wp->w_height - 2)		// not too far off
 	    {
 		i = plines_m_win(wp, wp->w_topline, wp->w_lines[0].wl_lnum - 1,
-									 TRUE);
+								wp->w_height);
 #ifdef FEAT_DIFF
 		// insert extra lines for previously invisible filler lines
 		if (wp->w_lines[0].wl_lnum != wp->w_topline)

src/misc1.c

@@ -497,12 +497,17 @@ plines_win_col(win_T *wp, linenr_T lnum, long column)
     return lines;
 }
 
+/*
+ * Return number of window lines the physical line range from "first" until
+ * "last" will occupy in window "wp". Takes into account folding, 'wrap',
+ * topfill and filler lines beyond the end of the buffer. Limit to "max" lines.
+ */
     int
-plines_m_win(win_T *wp, linenr_T first, linenr_T last, int limit_winheight)
+plines_m_win(win_T *wp, linenr_T first, linenr_T last, int max)
 {
     int		count = 0;
 
-    while (first <= last && (!limit_winheight || count < wp->w_height))
+    while (first <= last && count < max)
     {
 #ifdef FEAT_FOLDING
 	int	x;
@@ -531,9 +536,7 @@ plines_m_win(win_T *wp, linenr_T first, linenr_T last, int limit_winheight)
     if (first == wp->w_buffer->b_ml.ml_line_count + 1)
 	count += diff_check_fill(wp, first);
 #endif
-    if (limit_winheight && count > wp->w_height)
-	return wp->w_height;
-    return (count);
+    return MIN(max, count);
 }
 
     int

src/move.c

@@ -1455,7 +1455,7 @@ textpos2screenpos(
 
 	is_folded = hasFoldingWin(wp, lnum, &lnum, NULL, TRUE, NULL);
 #endif
-	row = plines_m_win(wp, wp->w_topline, lnum - 1, FALSE);
+	row = plines_m_win(wp, wp->w_topline, lnum - 1, INT_MAX);
 	// "row" should be the screen line where line "lnum" begins, which can
 	// be negative if "lnum" is "w_topline" and "w_skipcol" is non-zero.
 	row -= adjust_plines_for_skipcol(wp);
@@ -3219,12 +3219,18 @@ pagescroll(int dir, long count, int half)
 
 	int curscount = count;
 	// Adjust count so as to not reveal end of buffer lines.
-	if (dir == FORWARD)
+	if (dir == FORWARD
+		    && (curwin->w_topline + curwin->w_height + count > buflen
+#ifdef FEAT_FOLDING
+							|| hasAnyFolding(curwin)
+#endif
+	   ))
 	{
 	    int n = plines_correct_topline(curwin, curwin->w_topline, FALSE);
 	    if (n - count < curwin->w_height && curwin->w_topline < buflen)
-		n += plines_m_win(curwin, curwin->w_topline + 1, buflen, FALSE);
-	    if (n - count < curwin->w_height)
+		n += plines_m_win(curwin, curwin->w_topline + 1, buflen,
+						    curwin->w_height + count);
+	    if (n < curwin->w_height + count)
 		count = n - curwin->w_height;
 	}
 

src/popupwin.c

@@ -654,8 +654,8 @@ popup_show_curline(win_T *wp)
 	    wp->w_topline = wp->w_buffer->b_ml.ml_line_count;
 	while (wp->w_topline < wp->w_cursor.lnum
 		&& wp->w_topline < wp->w_buffer->b_ml.ml_line_count
-		&& plines_m_win(wp, wp->w_topline, wp->w_cursor.lnum, FALSE)
-								> wp->w_height)
+		&& plines_m_win(wp, wp->w_topline, wp->w_cursor.lnum,
+					    wp->w_height + 1) > wp->w_height)
 	    ++wp->w_topline;
     }
 

src/proto/misc1.pro

@@ -7,7 +7,7 @@ int plines_nofill(linenr_T lnum);
 int plines_win_nofill(win_T *wp, linenr_T lnum, int limit_winheight);
 int plines_win_nofold(win_T *wp, linenr_T lnum);
 int plines_win_col(win_T *wp, linenr_T lnum, long column);
-int plines_m_win(win_T *wp, linenr_T first, linenr_T last, int limit_winheight);
+int plines_m_win(win_T *wp, linenr_T first, linenr_T last, int max);
 int gchar_pos(pos_T *pos);
 int gchar_cursor(void);
 void pchar_cursor(int c);