patch 8.1.2140: "gk" and "gj" do not work correctly in number column

Problem:    "gk" and "gj" do not work correctly in number column.
Solution:   Allow for a negative "curswant". (Zach Wegner, closes #4969)
diff --git a/src/normal.c b/src/normal.c
index 1fddeae..50f7b31 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -2499,17 +2499,18 @@
 	    n = ((linelen - width1 - 1) / width2 + 1) * width2 + width1;
 	else
 	    n = width1;
-	if (curwin->w_curswant > (colnr_T)n + 1)
-	    curwin->w_curswant -= ((curwin->w_curswant - n) / width2 + 1)
-								     * width2;
+	if (curwin->w_curswant >= (colnr_T)n)
+	    curwin->w_curswant = n - 1;
       }
 
       while (dist--)
       {
 	if (dir == BACKWARD)
 	{
-	    if ((long)curwin->w_curswant > width2)
-		// move back within line
+	    if ((long)curwin->w_curswant >= width1)
+		// Move back within the line. This can give a negative value
+		// for w_curswant if width1 < width2 (with cpoptions+=n),
+		// which will get clipped to column 0.
 		curwin->w_curswant -= width2;
 	    else
 	    {
@@ -2557,6 +2558,12 @@
 		}
 		curwin->w_cursor.lnum++;
 		curwin->w_curswant %= width2;
+		// Check if the cursor has moved below the number display
+		// when width1 < width2 (with cpoptions+=n). Subtract width2
+		// to get a negative value for w_curswant, which will get
+		// clipped to column 0.
+		if (curwin->w_curswant >= width1)
+		    curwin->w_curswant -= width2;
 		linelen = linetabsize(ml_get_curline());
 	    }
 	}