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With latest changes for full textual diff, it would make sense that when you open a diff window with multiple files it would have none selected by default. How the new feature works, it would only require you to click on the output window to get the full diff rather than de-select the default selected file first. It would also bring the new feature forward as I doubt many people would even guess de-selecting would allow for a full diff. This could be a debatable UI change, so I'll just leave my opinion here.