fix(tui): harden highlight cache lifecycle

packages/tui/src/routes/session/index.tsx

@@ -96,6 +96,7 @@ import { findMessageBoundary, messageNavigationSlack } from "./message-navigatio
 import { stringWidth } from "../../util/string-width"
 import { useArgs } from "../../context/args"
 import { withTimestampedFallback } from "@opencode-ai/util/session-title-fallback"
+import { installSyntaxHighlightCache } from "../../util/syntax-highlight-cache"
 
 addDefaultParsers(parsers.parsers)
 
@@ -128,6 +129,7 @@ function use() {
 }
 
 export function Session() {
+  installSyntaxHighlightCache()
   const setEpilogue = useEpilogue()
   const clipboard = useClipboard()
   const writeExport = async (file: string, content: string) => {

packages/tui/src/util/syntax-highlight-cache.ts

@@ -0,0 +1,38 @@
+import { getTreeSitterClient, type TreeSitterClient } from "@opentui/core"
+
+const CACHE_SIZE = 500
+const installed = new WeakSet<TreeSitterClient>()
+
+export function installSyntaxHighlightCache() {
+  const client = getTreeSitterClient()
+  if (installed.has(client)) return
+  installed.add(client)
+  client.highlightOnce = cacheHighlights(client.highlightOnce.bind(client))
+}
+
+export function cacheHighlights(highlight: TreeSitterClient["highlightOnce"], capacity = CACHE_SIZE) {
+  const cache = new Map<string, ReturnType<TreeSitterClient["highlightOnce"]>>()
+
+  return (content: string, filetype: string) => {
+    const key = `${filetype}\0${content}`
+    const cached = cache.get(key)
+    if (cached) {
+      cache.delete(key)
+      cache.set(key, cached)
+      return cached
+    }
+
+    const result = highlight(content, filetype)
+    cache.set(key, result)
+    if (cache.size > capacity) cache.delete(cache.keys().next().value!)
+
+    void result
+      .then((value) => {
+        if (value.error && cache.get(key) === result) cache.delete(key)
+      })
+      .catch(() => {
+        if (cache.get(key) === result) cache.delete(key)
+      })
+    return result
+  }
+}

packages/tui/test/util/syntax-highlight-cache.test.ts

@@ -0,0 +1,69 @@
+import { describe, expect, test } from "bun:test"
+import { cacheHighlights } from "../../src/util/syntax-highlight-cache"
+
+describe("syntax highlight cache", () => {
+  test("reuses completed and in-flight highlights", async () => {
+    let calls = 0
+    const highlight = cacheHighlights(async () => {
+      calls++
+      return { highlights: [[0, 5, "keyword"]] }
+    })
+
+    const first = highlight("const", "typescript")
+    const second = highlight("const", "typescript")
+
+    expect(second).toBe(first)
+    expect(await second).toEqual({ highlights: [[0, 5, "keyword"]] })
+    expect(await highlight("const", "typescript")).toEqual({ highlights: [[0, 5, "keyword"]] })
+    expect(calls).toBe(1)
+  })
+
+  test("evicts least recently used highlights", async () => {
+    let calls = 0
+    const highlight = cacheHighlights(async () => {
+      calls++
+      return { highlights: [] }
+    }, 2)
+
+    await highlight("one", "text")
+    await highlight("two", "text")
+    await highlight("one", "text")
+    await highlight("three", "text")
+    await highlight("two", "text")
+
+    expect(calls).toBe(4)
+  })
+
+  test("retries failed highlights", async () => {
+    let calls = 0
+    const highlight = cacheHighlights(async () => {
+      calls++
+      if (calls === 1) return { error: "parser unavailable" }
+      return { highlights: [] }
+    })
+
+    await highlight("const", "typescript")
+    await highlight("const", "typescript")
+
+    expect(calls).toBe(2)
+  })
+
+  test("an evicted failure does not delete its replacement", async () => {
+    const pending = Promise.withResolvers<{ highlights: [] }>()
+    let calls = 0
+    const highlight = cacheHighlights(() => {
+      calls++
+      if (calls === 1) return pending.promise
+      return Promise.resolve({ highlights: [] })
+    }, 1)
+
+    const stale = highlight("one", "text")
+    await highlight("two", "text")
+    const current = highlight("one", "text")
+    pending.reject(new Error("parser unavailable"))
+
+    await expect(stale).rejects.toThrow("parser unavailable")
+    expect(highlight("one", "text")).toBe(current)
+    expect(calls).toBe(3)
+  })
+})