Re: (query-replace-regexp " +" " " nil) on single spaced text

[Richard Stallman]

    Please not that replacing a single space by a single space is *not* a
    noop.  It touches the buffer and changes its modification date.  It
    collapses all markers around the space, then moves them to before and
    behind the replaced space according to their insertion type (actually,
    does it really do that? Have not tried it).

replace-match has special code to avoid collapsing the markers in this
way.  Markers pointing previously before the replaced text remain
before; markers pointing after it remain after.

Any markers pointing into the middle of the replaced text
do get shoved to one end or the other.  But that case can't
happen when you replace just one character.