The window a fit has to work with. Normally the whole drawing area; with the Sheet menu pinned open over the left of it, what the menu leaves - which is what makes a pinned menu something to work beside rather than something in the way. Everything a fit does follows from this, so nothing below has to know the Sheet menu exists. Inset is how far in from the drawing area's left edge the usable part starts. Magnification does not care where the window begins, but the scroll does: it has to put the circuit past the menu and then half way across the rest.