compare-to-prior-version

Show changes from editing.

This command prompts for an edit count n, then compares the current version of the buffer with the version prior to the most recent n editing operations, displaying the differences in a new buffer. An "editing operation" means what one use of the undo-changes command would undo. If the edit count you enter is zero, the command instead compares the current version of the buffer with the version currently on disk. The comparison uses the format of the visual-diff command by default; set the compare-to-prior-version-style variable to use a different format.

More info:

Comparing Two Buffers
compare-to-prior-version-style

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compare-sorted-windows Command Reference compare-windows