Diff add/delete colors remain positional when the newer commit is on the left (follow-up to #196)

[pwiereng]

Found during the 2026-07-25 regression run (RT-12a). See docs/regression-report-2026-07-25.md.

Summary

DiffCodeViewer classifies lines purely from the diff library's positional added/removed
flags, so left is always red and right is always green regardless of which commit is newer.
When the newer commit is placed on the left, its unique lines render as red deletions and the
older commit's unique lines render as green additions.

Relationship to #196

#196 described exactly this inversion and was closed as completed by PR #201. PR #201 added
temporal awareness and affordances — referenceIsNewer / temporalOrderKnown in
useBrowseState.ts, older/newer chips on the panel headers, and a "Swap panels (put older on
left)" button — but it did not change the coloring. frontend/src/pages/Browse.tsx:604 passes
only leftContent / rightContent to <DiffCodeViewer>; no temporal props reach the component,
and DiffCodeViewer.tsx (lines ~58-88, computeSideBySideDiff, and getDiffBgColor ~503) still
derives add/remove entirely from position.

So the mitigation works — the user can see which side is which and can swap — but the underlying
colour semantics from #196 still reproduce.

Reproduction

File src/inxr2/adapters/external/treesitter/php_parser.py in the inxr2 test repo.

newer = 7f548aa2856220e8d163ca4065c01ae8a833b896  (2026-07-19 20:16 UTC)
older = ff935edf1867a52b9d58bb6660d253540de031a1  (2026-07-19 16:12 UTC)

git diff --numstat ff935edf 7f548aa -- <file>
  138   47

Going older → newer: 138 added, 47 removed.

# Case A - older on the left (default orientation)
/browse/inxr2/<file>?commit=7f548aa...&branch=main&diff=ff935edf...

# Case B - newer on the left (commit/diff swapped)
/browse/inxr2/<file>?commit=ff935edf...&branch=main&diff=7f548aa...

Counting row background colours via getComputedStyle:

Case	Left panel	Chips shown	Green rgba(80,161,79,.14)	Red rgba(228,86,73,.14)
A - older on left	ff935ed	newer / older	138	47
B - newer on left	7f548aa	newer / older	47	138

In Case B the gutter markers agree with the colours (+ appears on the older side).

Expected

The 138 lines that exist only in the newer commit should be green additions in both
orientations; the 47 older-only lines should be red. This is the criterion RT-12a in
docs/regression-tests.md asserts: "Colors are correct regardless of which side (left/right)
each commit appears on."

Notes on severity / scope

Not critical. The default orientation the UI produces is Case A, which is correct, and the
older/newer chips plus the swap button mean a user is not left without a signal. But a reader
who lands on a swapped diff sees additions labelled as deletions.

Worth deciding explicitly whether the intended contract is:

• temporal (flip added/removed when referenceIsNewer, i.e. option A/C from Diff viewer colors are reversed when left commit is newer than right commit #196), or
• positional (ordinary git diff semantics, relying on the chips) — in which case RT-12a in
  docs/regression-tests.md should be rewritten to match, since it currently asserts temporal.

Affected files

• frontend/src/components/DiffCodeViewer/DiffCodeViewer.tsx - computeSideBySideDiff(), getDiffBgColor()
• frontend/src/pages/Browse.tsx:604 - does not forward temporal context
• frontend/src/hooks/useBrowseState.ts:112-145 - computes referenceIsNewer / temporalOrderKnown (used for chips only)