is a variable defined in `
Its value is
Size of string from one window that is searched in second window.
Small number makes difference regions more fine-grained, but it
may fail by finding the wrong match. The bigger number makes
difference regions more coarse-grained.
The default value 32 is good for the most cases.
You can customize this variable.
This variable was introduced, or its default value was changed, in
version 22.1 of Emacs.