[OS X TeX] TeXShop previews in portrait mode

[AES]

I'm sorry -- I'm just not grasping how to preview portrait pages 
(e.g., slides) in TeXShop, in Plain TeX.

The test input for a prototypical slide is

\magnification =\magstepN
\hsize = 10 truein
\vsize = 7 truein
\nopagenumbers
\hrule
\vfill
\centerline{SLIDE TITLE}
\bigskip
\centerline{Some slide content}
\vfill
\vfill
\hrule
\eject

where the \hrules are inserted temporarily to define the typeset 
area, and the value N  of the \magstep is manually adjusted during 
composition to suit the amount and type of content on the slide.

The objectives are

(a)  A preview of fixed size such that the typeset area (10 wide X 7 
high) -- or if you like the page size, 11 X 8.5 -- comes close to 
filling a preview window that's been adjusted to nearly fill my 
screen (so I can see the largest preview of the slide); and

(b)  To have this preview window *stay* that way when I change N and 
re-typeset.

If I try to use Preview >> Magnification >> Fit to Window:

(a) I can't use the whole width of the preview window, only about 60% 
of it, since the gray page boundary doesn't change to portrait; and

(b)  When I change N and re-typeset, the entire slide either shrinks 
in proportion or expands in proportion -- until part of it becomes 
hidden on the right.

If I try to use Preview >> Magnification >> Fit to Width with a 
suitable scale setting, when I increase N and re-typeset the entire 
slide jumps upward (scroll bar jumps down), putting most of the slide 
off the top of the window.

Is there a solution for this?  (in *Plain* TeX and TeXShop 1.43)

Thanks . . .
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