patch 8.1.2029: cannot control 'cursorline' highlighting well

Problem:    Cannot control 'cursorline' highlighting well.
Solution:   Add "screenline". (Christian Brabandt, closes #4933)
diff --git a/src/main.c b/src/main.c
index 6fe581c..d1d9e32 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1255,12 +1255,28 @@
 	    update_topline();
 	    validate_cursor();
 
+#ifdef FEAT_SYN_HL
+	    if (curwin->w_p_cul && curwin->w_p_wrap
+				&& (curwin->w_p_culopt_flags & CULOPT_SCRLINE))
+		must_redraw = NOT_VALID;
+#endif
+
 	    if (VIsual_active)
-		update_curbuf(INVERTED);/* update inverted part */
+		update_curbuf(INVERTED); // update inverted part
 	    else if (must_redraw)
 	    {
-		mch_disable_flush();	/* Stop issuing gui_mch_flush(). */
-		update_screen(0);
+		mch_disable_flush();	// Stop issuing gui_mch_flush().
+#ifdef FEAT_SYN_HL
+		// Might need some more update for the cursorscreen line.
+		// TODO: can we optimize this?
+		if (curwin->w_p_cul
+			&& curwin->w_p_wrap
+			&& (curwin->w_p_culopt_flags & CULOPT_SCRLINE)
+			&& !char_avail())
+		    update_screen(VALID);
+		else
+#endif
+		    update_screen(0);
 		mch_enable_flush();
 	    }
 	    else if (redraw_cmdline || clear_cmdline)