The source of this "problem" is that we don't show any kind of adorner for the DesignWidth/DesignHeight, and the "Fit All" command is fitting the design size, not the root size.
I think it would be more clear and not perceived as an error if we render some adorner for the design size.

There's also a command, the 'F' shortcut, for fitting the selected element, so you can always select the root and fit that into the stage.

designadorner.png (104,704 bytes)   

designadorner.png (104,704 bytes)   

Thanks for looking into this.

So, in my opinion, the "Fit All" command should actually more or less do what the Select Element + "F" currently do, but systematically do it as if the "Page Root" was selected.
Out of all the Zoom Modes, this is actually the one that I consider the most valuable, to the point where I would gladly sacrifice one of the other ones if it meant having this mode act this way.

So in summary, perhaps something like this:

1) "Fit All": Sets Stage to Maximum size that it can fit in the Viewport. (as described in intro above)

2) Element Selected + "F" Key Shortcut: Sets the individual Element to the Maximum size that it can fit in the Viewport.

3) "Reset Zoom": Sets Stage to either Actual Design Height/Design Width Resolution (including your suggestion on adding the adorners) OR Sets Stage to Actual Height/Actual Width Resolution.
What I would be most curious about here, is what is considered higher value between 'DesignTime' and 'Actual' in the context of editing files in a non-Play mode, and then, if there is any potential, or value in having this Reset Zoom perform different functions whether you are playing or not.