Oh, starting in 12.9, we totally replaced the side-by-side comparison window because the old one was too slow and sometimes never finished for large files. If you click "Enable Merge", it should produce a merged source at the bottom, and if you don't like which side it used for the merge on a particular difference, you can select the line at the top and click either the "Use right side in merge" or "Use left side in merge" toolbar buttons.
↧