The small square preview of surface shaders has 0,0 in the middle, except that that is the point where imported and generated terrains start by default, so to see the whole terrain one has to zoom out to 16 km(default is 1 km), when it fills about a quarter of a very small image, since panning is not possible on this.
You can use the ALT+mouse click to move the image around, but this doesn't change the view.
This has made it difficult to visualize an image mask's placement and the general distribution of textures.
On the contrary, the (unzoomable) terrain preview shows the whole generated terrain and (I suppose) the 0 to 10000 area of a fractal one.
Setting the shader preview to 0,0 at the lower left corner with the ability to pan would help a lot.
Helen