Painting with Gradients or Textures on Bitmap Layers
When using the Paint tool to fill a zone in a bitmap layer with a gradient or texture, a temporary vector object in the shape of the fill is created, and contains the fill. This allows you to use the Edit Gradient/Texture tool on this vector shape to tweak your gradient or texture so that it has the intended position, angle and scale. Then, the next modification you make to your bitmap layer—be it with a drawing tool, or by pasting artwork—will cause the gradient/texture to merge with your artwork, making it no longer possible to tweak it with the Edit Gradient/Texture tool from that point on. Hence, it is important to make sure your gradients or textures look exactly as you intend them to before continuing to work on your drawing.