Diffs: long lines are hard to scroll horizontally without reaching the end of the file #1324

Problem

In a long code diff, the horizontal scrollbar sits at the very bottom of the whole diff, so a reader who needs to see the end of a long line has to scroll to the end of the file first (or use Shift+wheel / a trackpad, which many readers don't know about). Raised in #1127.

Direction

Solve it at the Pierre diff layer (introduced in #1277) rather than with a second scroll controller: e.g. an optional line-wrap toggle for diffs, or a sticky horizontal scrollbar if Pierre supports one. #1127's measurements and e2e scenarios are a useful reference.