R1:37ffb71c4d5e

In source views, wrap display tabs in "user-select: all" to improve cursor selection behavior

Summary:
Ref T2495. See PHI1814. Currently, Phabricator replaces tabs with spaces when rendering diffs.

This may or may not be the best behavior in the long term, but it gives us more control over expansion of tabs than using tab literals.

However, one downside is that you can use your mouse cursor to select "half a tab", and can't use your mouse cursor to distinguish between tabs and spaces. Although you…