I searched for this but didn't see it as a feature that exists or has been requested before. So hopefully this is copacetic.
Currently when you open an image and add more images to it as layers, the first image is your background. When you enlarge the canvas, the background of the new area that's been added has the background color in your palette.
Problem: If you want to have all of your layers transparent but you don't want to see that checkboard as the background, you need to create a new layer, move it to the bottom, and color it white. But if you enlarge the canvas, your new background doesn't take the background color in your palette -- your first image layer does. So you need to delete the color from the first layer and add it to your chosen background layer.
One workaround -- which I read about here and hope to remember for next time! -- is to copy the first image to a new layer and make that original first image layer into your background (flood it with white).
Request: What I suggest instead is the ability to select which layer is the background (meaning, it will fill with the background palette color when you enlarge the canvas). Perhaps a checkbox in the layer properties window? Also, a little icon on the layer palette (or is it called window?) that indicates which layer is the background would be nice.