xzoom (magnify part of X display, with real-time updates)

Like xmag, xzoom magnifies a section of the X display. Xzoom is
different because it will continuously update the magnified area as
the display changes. It is fast enough to enlarge small animations,
for example. It can also mirror or rotate a part of the screen.

Xzoom displays in its window a magnified area of the X11 display. The
user can interactively change the zoomed area, the window size,
magnification (optionally different magnification for X and Y axes) or
rotate or mirror the image. See the man page for the list of keyboard
and mouse commands.

If you don't like the "rubber-band" frame xzoom uses when dragging with
the mouse, you can disable it by exporting FRAME=no in the script's
environment (sorry, it's a compile-time option only).