patch 8.1.0374: moving the cursor is slow when 'relativenumber' is set

Problem:    Moving the cursor is slow when 'relativenumber' is set.
Solution:   Only redraw the number column, not all lines.

src/move.c

@@ -145,21 +145,25 @@ redraw_for_cursorline(win_T *wp)
 # endif
 	    )
     {
+	if (wp->w_p_rnu)
+	    // win_line() will redraw the number column only.
+	    redraw_win_later(wp, VALID);
 #ifdef FEAT_SYN_HL
-	if (!wp->w_p_rnu && wp->w_redr_type <= VALID && last_cursorline != 0)
+	if (wp->w_p_cul)
 	{
-	    // "last_cursorline" may be set for another window, worst case we
+	    if (wp->w_redr_type <= VALID && last_cursorline != 0)
-	    // redraw too much.  This is optimized for moving the cursor around
+	    {
-	    // in the same window.
+		// "last_cursorline" may be set for another window, worst case
+		// we redraw too much.  This is optimized for moving the cursor
+		// around in the same window.
 		redrawWinline(wp, last_cursorline, FALSE);
 		redrawWinline(wp, wp->w_cursor.lnum, FALSE);
 		redraw_win_later(wp, VALID);
 	    }
 	    else
-#endif
 		redraw_win_later(wp, SOME_VALID);
-#ifdef FEAT_SYN_HL
 	    last_cursorline = wp->w_cursor.lnum;
+	}
 #endif
     }
 }

src/screen.c

@@ -132,7 +132,7 @@ static void fold_line(win_T *wp, long fold_count, foldinfo_T *foldinfo, linenr_T
 static void fill_foldcolumn(char_u *p, win_T *wp, int closed, linenr_T lnum);
 static void copy_text_attr(int off, char_u *buf, int len, int attr);
 #endif
-static int win_line(win_T *, linenr_T, int, int, int nochange);
+static int win_line(win_T *, linenr_T, int, int, int nochange, int number_only);
 static int char_needs_redraw(int off_from, int off_to, int cols);
 static void draw_vsep_win(win_T *wp, int row);
 #ifdef FEAT_STL_OPT
@@ -971,7 +971,8 @@ update_single_line(win_T *wp, linenr_T lnum)
 		start_search_hl();
 		prepare_search_hl(wp, lnum);
 # endif
-		win_line(wp, lnum, row, row + wp->w_lines[j].wl_size, FALSE);
+		win_line(wp, lnum, row, row + wp->w_lines[j].wl_size,
+								 FALSE, FALSE);
 # if defined(FEAT_SEARCH_EXTRA)
 		end_search_hl();
 # endif
@@ -1881,7 +1882,7 @@ win_update(win_T *wp)
 	/*
 	 * Update a line when it is in an area that needs updating, when it
 	 * has changes or w_lines[idx] is invalid.
-	 * bot_start may be halfway a wrapped line after using
+	 * "bot_start" may be halfway a wrapped line after using
 	 * win_del_lines(), check if the current line includes it.
 	 * When syntax folding is being used, the saved syntax states will
 	 * already have been updated, we can't see where the syntax state is
@@ -2140,7 +2141,8 @@ win_update(win_T *wp)
 		/*
 		 * Display one line.
 		 */
-		row = win_line(wp, lnum, srow, wp->w_height, mod_top == 0);
+		row = win_line(wp, lnum, srow, wp->w_height,
+							  mod_top == 0, FALSE);
 
 #ifdef FEAT_FOLDING
 		wp->w_lines[idx].wl_folded = FALSE;
@@ -2177,7 +2179,14 @@ win_update(win_T *wp)
 	}
 	else
 	{
-	    /* This line does not need updating, advance to the next one */
+	    if (wp->w_p_rnu)
+	    {
+		// 'relativenumber' set: The text doesn't need to be drawn, but
+		// the number column nearly always does.
+		(void)win_line(wp, lnum, srow, wp->w_height, TRUE, TRUE);
+	    }
+
+	    // This line does not need to be drawn, advance to the next one.
 	    row += wp->w_lines[idx++].wl_size;
 	    if (row > wp->w_height)	/* past end of screen */
 		break;
@@ -3058,7 +3067,8 @@ win_line(
     linenr_T	lnum,
     int		startrow,
     int		endrow,
-    int		nochange UNUSED)	/* not updating for changed text */
+    int		nochange UNUSED,	// not updating for changed text
+    int		number_only)		// only update the number column
 {
     int		col = 0;		/* visual column on screen */
     unsigned	off;			/* offset in ScreenLines/ScreenAttrs */
@@ -3253,6 +3263,8 @@ win_line(
     row = startrow;
     screen_row = row + W_WINROW(wp);
 
+    if (!number_only)
+    {
 	/*
 	 * To speed up the loop below, set extra_check when there is linebreak,
 	 * trailing white space and/or syntax processing to be done.
@@ -3460,6 +3472,7 @@ win_line(
 	    area_highlighting = TRUE;
 	    attr = HL_ATTR(HLF_I);
 	}
+    }
 
 #ifdef FEAT_DIFF
     filler_lines = diff_check(wp, lnum);
@@ -3504,7 +3517,7 @@ win_line(
     ptr = line;
 
 #ifdef FEAT_SPELL
-    if (has_spell)
+    if (has_spell && !number_only)
     {
 	/* For checking first word with a capital skip white space. */
 	if (cap_col == 0)
@@ -3564,7 +3577,7 @@ win_line(
 	v = wp->w_skipcol;
     else
 	v = wp->w_leftcol;
-    if (v > 0)
+    if (v > 0 && !number_only)
     {
 #ifdef FEAT_MBYTE
 	char_u	*prev_ptr = ptr;
@@ -3707,7 +3720,7 @@ win_line(
      */
     cur = wp->w_match_head;
     shl_flag = FALSE;
-    while (cur != NULL || shl_flag == FALSE)
+    while ((cur != NULL || shl_flag == FALSE) && !number_only)
     {
 	if (shl_flag == FALSE)
 	{
@@ -4068,13 +4081,16 @@ win_line(
 	    }
 	}
 
-	/* When still displaying '$' of change command, stop at cursor */
+	// When still displaying '$' of change command, stop at cursor.
-	if (dollar_vcol >= 0 && wp == curwin
+	// When only displaying the (relative) line number and that's done,
+	// stop here.
+	if ((dollar_vcol >= 0 && wp == curwin
 		   && lnum == wp->w_cursor.lnum && vcol >= (long)wp->w_virtcol
 #ifdef FEAT_DIFF
 				   && filler_todo <= 0
 #endif
 		)
+		|| (number_only && draw_state > WL_NR))
 	{
 	    screen_line(screen_row, wp->w_wincol, col, -(int)wp->w_width,
 						    HAS_RIGHTLEFT(wp->w_p_rl));

src/version.c

@@ -794,6 +794,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    374,
 /**/
     373,
 /**/