is a variable defined in `
Its value is
Non-nil means compare-windows highlights the differences.
The value t removes highlighting immediately after invoking a command
other than `compare-windows'.
The value `persistent' leaves all highlighted differences. You can clear
out all highlighting later with the command `compare-windows-dehighlight'.
You can customize this variable.
This variable was introduced, or its default value was changed, in
version 22.1 of Emacs.