You have to have the pointer in exactly the right place and you also have to be careful not to move the pointer out of place when you click the button. Like I said, the area is only about one pixel wide. The mouse pointer normally points to the upper left but if you place it in just the right spot it will flip over to point to the upper right. Then you can select a full line with one click or hold and scroll to select a group of lines. It's the same feature you see on MS Word when you click in page area to the left of the text.
