To understand why this is a problem, remember that screen updates are
calculated between two representations of the entire display. The
documentation says that when you refresh a window, it is first copied
- to to the virtual screen, and then changes are calculated to update
+ to the virtual screen, and then changes are calculated to update
the physical screen (and applied to the terminal). But "copied to" is
not very specific, and subtle differences in how copying works can
produce different behaviors in the case where two overlapping windows