When using the mouse wheel to zoom the image you can hold ctrl to get it smaller than fitting to the screen. That’s handy to draw masks over the image border.
I don’t use Darktable myself (as I’m making my own photo editor), but what’s the reasoning behind preventing the user from zooming out in the first place?
(Filmulator at the moment always generates the full size image, so you’re free to pan and zoom around. Zoom out until the whole image is a pixel, or zoom in until all you can see is one pixel. Double click to instantly get back to either 100% or fit-to-screen, whichever is closer, unless you’re between them in which case it goes to 100%.)