1. Click on a line in the diff viewer (e.g. when previewing a commit in the log/history tab)
2. Ctrl+C
3. Paste in any IDE/text editor
4. Any tabs present in the original source file have been converted to four spaces

It would be very useful for any lines copied from the diff viewer to have their original whitespace preserved. I often find myself wanting to grab a line I deleted a few commits ago, and do so by selecting the commit where I removed it and copying it out of the diff viewer. Because the diff viewer converts tabs to spaces, I have to fix the whitespace every time.