How to fill transparent area left by resize in Gimp 2.8

Command in menu Image -> Flatten Image will fill transparent area with current background color selection (as hinted by pop-up help when you select it - "... remove transparency"), making exactly what you asked in one simple step (tested in Gimp 2.8).

You cannot fill outside the boundaries of the layer. If initially your layer is the size of the canvas and you scale it down, it will be smaller than the canvas so there are areas around it that you won't be able to fill. Two solutions:

1. Add a canvas-size layer below it and paint that layer
2. Use Layer>Layer to image size to enlarge the layer so that it fills the canvas
3. (*) Use Image>Fit canvas to layers to shrink the canvas around the layer so the fill isn't needed.

(*) The Spanish Inquisition made me do it.