[fhomess]

The yellow and black "border" in the second image of your original post is the layer border. By resizing all layers when expanding the canvas size, you simply expand the layer to the canvas. If you forget to click that check box, there's also an option in the Layer menu to increase an individual later to the image size. Depending on how complex your image layers are, it's often nice to have a layer that is sized just to the content in it.