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The current app editor is pretty good -- but it could be better. We monitor our work on a vertical representation of a smartphone screen but, if you like to use a moderately large font, or are working on a small screen, the allocation of vertical space can be problematic. I often find that the top bar is hidden as shown below:
One way to improve the situation would be to allow us to toggle closed certain spaces that we don't need. You already let us do this horizontally (by hiding or showing the smartphone screen image) but not vertically. Even if you don't allow such a toggle, I would think that space could be used more efficiently. The area I've highlighted in blue, for example, could be a good deal thinner.
Another strategy might be simply not to allow the top bar on the app image to be hidden as it is in my image.
That's the idea. The editing interface is certainly serviceable as it is now, but I think the kind of reallocation and improved control of space that I've described here would make it more comfortable, particularly for visually impaired creators.