patch 8.2.4644: redrawing too often when 'relativenumber' is set

Problem:    Redrawing too often when 'relativenumber' is set.
Solution:   Only redraw when the cursor line changed. (Lewis Russell,
            closes #10040)

src/change.c

@@ -641,7 +641,7 @@ changed_common(
 		set_topline(wp, wp->w_topline);
 #endif
 	    // Relative numbering may require updating more.
-	    if (wp->w_p_rnu)
+	    if (wp->w_p_rnu && xtra != 0)
 		redraw_win_later(wp, SOME_VALID);
 #ifdef FEAT_SYN_HL
 	    // Cursor line highlighting probably need to be updated with

src/drawscreen.c

@@ -2507,11 +2507,11 @@ win_update(win_T *wp)
 	}
 	else
 	{
-	    if (wp->w_p_rnu)
+	    if (wp->w_p_rnu && wp->w_last_cursor_lnum_rnu != wp->w_cursor.lnum)
 	    {
 #ifdef FEAT_FOLDING
-		// 'relativenumber' set: The text doesn't need to be drawn, but
-		// the number column nearly always does.
+		// 'relativenumber' set and the cursor moved vertically: The
+		// text doesn't need to be drawn, but the number column does.
 		fold_count = foldedCount(wp, lnum, &win_foldinfo);
 		if (fold_count != 0)
 		    fold_line(wp, fold_count, &win_foldinfo, lnum, row);
@@ -2553,6 +2553,7 @@ win_update(win_T *wp)
     // update w_last_cursorline.
     wp->w_last_cursorline = wp->w_p_cul ? wp->w_cursor.lnum : 0;
 #endif
+    wp->w_last_cursor_lnum_rnu = wp->w_p_rnu ? wp->w_cursor.lnum : 0;
 
 #ifdef FEAT_VTP
     // Rewrite the character at the end of the screen line.

src/structs.h

@@ -3465,6 +3465,9 @@ struct window_S
     colnr_T	w_old_visual_col;   // last known start of visual part
     colnr_T	w_old_curswant;	    // last known value of Curswant
 
+    linenr_T    w_last_cursor_lnum_rnu;  // cursor lnum when 'rnu' was last
+					 // redrawn
+
     lcs_chars_T	w_lcs_chars;	    // 'listchars' characters
 
     /*

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4644,
 /**/
     4643,
 /**/