fix(session-ui): render large diffs as plain text in the worker pool (#46772)

packages/session-ui/performance/highlighting/README.md

@@ -25,8 +25,9 @@ mechanism, not CPU time or desktop memory. Full reconstructed content is checked
 separately after timing. No forced GC or machine-dependent thresholds are used.
 
 `HIGHLIGHT_CORRECTNESS=1` runs the deterministic `*.correctness.ts` checks against
-the same fixture instead of the timed benchmarks: cache reuse across remounts and
-invalidation on content, theme, and worker option changes.
+the same fixture instead of the timed benchmarks: cache reuse across remounts,
+invalidation on content, theme, and worker option changes, and plain rendering of
+large diffs with complete reconstructed content.
 
 Optional output settings: `HIGHLIGHT_RESULTS`, `HIGHLIGHT_SCREENSHOTS`,
 `OPENCODE_PERFORMANCE_RUN_ID`, and `OPENCODE_PERFORMANCE_TRACE_DIR` (separate

packages/session-ui/performance/highlighting/plain.correctness.ts

@@ -0,0 +1,19 @@
+import { expect, test } from "@playwright/test"
+import type {} from "./viewer"
+
+test("renders large diffs as plain text without changing ordinary highlighting", async ({ page }) => {
+  await page.goto("/?large")
+  await page.waitForFunction(() => !!window.highlighting)
+  const large = await page.evaluate(() => window.highlighting.mount(1))
+  expect(large.options).toMatchObject({ lineDiffType: "none", maxLineDiffLength: 0, tokenizeMaxLineLength: 1 })
+  expect(large.syntaxSpans).toBe(0)
+  await expect(page.locator('[data-line="11"][data-line-type="change-addition"]')).toContainText("status: 201")
+  expect(await page.evaluate(() => window.highlighting.contents()!.after === window.highlighting.input.after[0])).toBe(true)
+
+  await page.goto("/")
+  await page.waitForFunction(() => !!window.highlighting)
+  const ordinary = await page.evaluate(() => window.highlighting.mount(1))
+  expect(ordinary.options).toMatchObject({ lineDiffType: "word-alt", maxLineDiffLength: 1000, tokenizeMaxLineLength: 1000 })
+  expect(ordinary.syntaxSpans).toBeGreaterThan(0)
+  await expect(page.locator('[data-line="11"][data-line-type="change-addition"]')).toContainText("status: 201")
+})

packages/session-ui/src/pierre/worker.ts

@@ -22,6 +22,9 @@ function createPool(lineDiffType: "none" | "word-alt") {
     {
       theme: "OpenCode",
       lineDiffType,
+      // Pierre renders with the pool's options, not the viewer's, whenever the pool works. The "none" pool only
+      // serves diffs above the large-file threshold, so it carries the plain-text fallback the viewer requests.
+      ...(lineDiffType === "none" && { maxLineDiffLength: 0, tokenizeMaxLineLength: 1 }),
       preferredHighlighter: "shiki-wasm",
     },
   )