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tickbox variable: $undoeditflag -- toggle enable undo
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keyboard equiv: cmd-z OR ctrl-left-click in image window

The "un" tickbox toggles whether a 4-step image
edit undo function is enabled (default=ON).  This
can undo any series of sequential edits, up to 4
in a row.  There is also a 1-step redo.

This slightly slows down a series of rapid single
right- or middle-click edits (virtually no effect
on a right- or middle-click-drag), and can be
turned off if individual-click edit interface
response speed is critical (and no undo is OK).

A cmd-z *OR* a ctrl-left-click on the image
window undoes the latest right- or middle-click
edit or right/middle click-drag edit.  cmd-Z for
1-step redo.

