We know when we have reached the last difference between the two files, because the down arrow in the toolbar (pointed at by our big red arrow 1) turns dark.

Notice that the up arrow on the toolbar is still yellow.  Clicking on this toolbar up arrow can take you to the next difference above the current position.  Because we found that we had to leave some file differences alone, we expected that this up arrow would remain yellow.

Once we click on the yellow arrow in the left gutter that copies the last difference to the file on the right...

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