test(diff): automate DiffView/CodeBlock horizontal-scroll coverage (closes #21) (#101)

Turns the manual QA ask ("verify DiffView + CodeBlock horizontal-scroll
on-device with a populated diff") into two automated layers:

1. Unit (deterministic, runs in `npm test` now): extracted the shared
   ScrollView props into src/lib/scroll-config.ts (WIDE_CONTENT_SCROLL_CONFIG)
   so DiffView.tsx and CodeBlock.tsx spread the SAME plain object their tests
   assert on — no react-native-renderer needed. Added a source-scan
   regression test (wide-content-scroll.regression.test.ts) that fails if
   either component loses its ScrollView wiring or reintroduces
   numberOfLines truncation.

2. E2E (Maestro): .maestro/flows/diff-scroll.yaml opens a session with a
   pre-seeded wide edit-diff tool call and a wide fenced code block, then
   swipes each horizontal ScrollView left and asserts the off-screen marker
   text becomes visible. mock-opencode-server.ts gained a --seed-diff mode
   that serves this session via GET /session/:id/message (pre-existing
   history), not SSE — issue #90 (a separate SSE-render bug) is being fixed
   independently, and this flow must not depend on it landing first. Wired
   the new flow + port 4100 into run-e2e-flows.sh and activation-e2e.yml.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/lib/scroll-config.test.ts

@@ -0,0 +1,26 @@
+import { test } from "node:test"
+import assert from "node:assert/strict"
+import { WIDE_CONTENT_SCROLL_CONFIG } from "./scroll-config.ts"
+
+// GitHub issue #21: DiffView + CodeBlock must render wide content in a
+// horizontally-scrollable container (not wrapped, not truncated). Both
+// components spread WIDE_CONTENT_SCROLL_CONFIG onto their ScrollView (see
+// src/components/chat/DiffView.tsx and src/components/markdown/CodeBlock.tsx)
+// so asserting on this object here is asserting on the actual runtime props,
+// not a parallel copy that can drift out of sync.
+
+test("wide-content scroll config enables horizontal scrolling", () => {
+  assert.equal(WIDE_CONTENT_SCROLL_CONFIG.horizontal, true)
+})
+
+test("wide-content scroll config shows the horizontal scroll indicator", () => {
+  // Regression guard: a container that scrolls but hides its indicator is
+  // easy to mistake for content that simply doesn't overflow. Keep the
+  // indicator visible so on-device QA (and screenshots) can tell scrollable
+  // content apart from clipped/truncated content.
+  assert.equal(WIDE_CONTENT_SCROLL_CONFIG.showsHorizontalScrollIndicator, true)
+})
+
+test("wide-content scroll config has no unexpected keys", () => {
+  assert.deepEqual(Object.keys(WIDE_CONTENT_SCROLL_CONFIG).sort(), ["horizontal", "showsHorizontalScrollIndicator"])
+})

src/lib/scroll-config.ts

@@ -0,0 +1,18 @@
+// Pure (no React Native imports) horizontal-scroll configuration shared by
+// src/components/chat/DiffView.tsx and src/components/markdown/CodeBlock.tsx.
+//
+// GitHub issue #21: wide diff lines and wide code-block lines must render in
+// a horizontally-scrollable container instead of being wrap-broken or
+// truncated with `numberOfLines`. Centralizing the actual runtime props in
+// one plain object lets that decision be unit-tested with node:test (no React
+// Native renderer needed) while both components spread the SAME object onto
+// their ScrollView, so the test and the real components can't drift apart.
+export interface HorizontalScrollConfig {
+  horizontal: true
+  showsHorizontalScrollIndicator: boolean
+}
+
+export const WIDE_CONTENT_SCROLL_CONFIG: HorizontalScrollConfig = {
+  horizontal: true,
+  showsHorizontalScrollIndicator: true,
+}