Ref T13161. See PHI723. Our whitespace handling is based on whitespace flags like diff -bw, mostly just for historical reasons: long ago, the easiest way to minimize the visual impact of indentation changes was to literally use diff -bw.

However, this approach is very coarse and has a lot of problems, like detecting "ab" -> "a b" as "only a whitespace change" even though this is always semantic. It also causes problems in YAML, Python, etc. Over time, we've added a lot of stuff to mitigate the downsides to this approach.

We also no longer get any benefits from this approach being simple: we need faithful diffs as the authoritative source, and have to completely rebuild the diff to diff -bw it. In the UI, we have a "whitespace mode" flag. We have the "whitespace matters" configuration.

I think ReviewBoard generally has a better approach to indent depth changes than we do (see T13161) where it detects them and renders them in a minimal way with low visual impact. This is ultimately what we want: reduce visual clutter for depth-only changes, but preserve whitespace changes in strings, etc.

Move toward detecting and rendering indent depth changes. Followup work:

• These should get colorblind colors and the design can probably use a little more tweaking.
• The OneUp mode is okay, but could be improved.
• Whitespace mode can now be removed completely.
• I'm trying to handle tabs correctly, but since we currently mangle them into spaces today, it's hard to be sure I actually got it right.