Layers in GIMP can be smaller than the image they are part of. So if you try to edit a layer outside of it's boundaries you don't get anything. Pretty much all editors do this as an optimisation (no point storing an entire layer if it's almost entirely empty transparent pixels), but GIMP is unusual in exposing this to the user and expecting them to increase the layer size manually. In other editors the layer is conceptually the full size of the image and these boundaries are managed automatically. (In fact, usually the layers are unbounded in size and can expand to be larger than the canvas. Very useful if e.g. you are importing a background image and want to be able to shift it around a bit)
Not just smaller, but you can even have them be larger than the image. Then you can move the layer around and have the image essentially acting as a cropping window into that layer. I most commonly experience that when I've imported/pasted in a larger image as a layer
Nice solution would be having this boundary stretching automatically as proposed in this thread but also exposing 'bounded layers' as separate tool like frames in Indesign.
