If that’s it, you’ll probably need to adjust the center/origin to be in the middle of the left edge of the object, so it doesn’t slide up or down as expanding.
Moving the origin would definitely be the easiest way. Should you want to use a tiled sprite, this would work. I tested it with 2 slider controls. I used NewSprite for a reference position and a tiled sprite for the line. Note: it uses set centered position.
It just occurred to me that put around object would also work.