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to nil when changing the window's start.  Don't change the
window's start when its top position hasn't changed.  If we do,
this will set the window's optional_new_start, which act's like a
force_start during redisplay with C-x 1 M-> under particular
circumstances (see report from Per Starback to emacs-pretest-bug
from 2000-12-13.).
(Fdelete_other_windows): Set window's window_end_valid
to nil when changing the window's start.