feat(web): virtualize sandbox file preview and handle empty files

Wraps the Pierre File renderer in WorkerPoolContextProvider + Virtualizer
so long file previews scroll efficiently and reuse the shared highlighter
worker pool. Adds a content-aware sandbox cacheKey so previews don't
re-highlight unchanged content. Empty files now render an "Empty file"
state instead of mounting an empty File component.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/fabro-web/app/routes/run-sandbox.test.tsx

@@ -58,6 +58,12 @@ mock.module("@pierre/trees", () => ({ themeToTreeStyles: () => ({}) }));
 mock.module("@pierre/theme/pierre-dark", () => ({ default: {} }));
 mock.module("@pierre/diffs/react", () => ({
   File: () => <div data-test-id="pierre-file-stub" />,
+  Virtualizer: ({ children }: { children?: ReactNode }) => (
+    <div data-test-id="pierre-virtualizer-stub">{children}</div>
+  ),
+  WorkerPoolContextProvider: ({ children }: { children?: ReactNode }) => (
+    <div data-test-id="pierre-worker-pool-stub">{children}</div>
+  ),
 }));
 
 const { default: RunSandbox, formatBytesAsMemory, normalizeSandboxMode } =

apps/fabro-web/app/routes/run-sandbox/filesystem-panel.test.tsx

@@ -25,6 +25,9 @@ let filesState: FilesQueryState = makeEmptyFilesState();
 let lastFileArgs: { id: string | undefined; path: string | null | undefined } | null = null;
 let fileState: FileQueryState = makeEmptyFileState();
 let lastTreeOptions: CapturedTreeOptions | null = null;
+const pierreFileCalls: Array<{ file: { name: string; contents: string; cacheKey?: string } }> = [];
+const providerCalls: any[] = [];
+const virtualizerCalls: any[] = [];
 
 function makeEmptyFilesState(): FilesQueryState {
   return {
@@ -81,11 +84,22 @@ mock.module("@pierre/theme/pierre-dark", () => ({
 }));
 
 mock.module("@pierre/diffs/react", () => ({
-  File: (props: { file: { name: string; contents: string } }) => (
-    <div data-test-id="pierre-file" data-file-name={props.file.name}>
-      {props.file.contents}
-    </div>
-  ),
+  File: (props: { file: { name: string; contents: string; cacheKey?: string } }) => {
+    pierreFileCalls.push(props);
+    return (
+      <div data-test-id="pierre-file" data-file-name={props.file.name}>
+        {props.file.contents}
+      </div>
+    );
+  },
+  Virtualizer: (props: any) => {
+    virtualizerCalls.push(props);
+    return <div data-test-id="pierre-virtualizer">{props.children}</div>;
+  },
+  WorkerPoolContextProvider: (props: any) => {
+    providerCalls.push(props);
+    return <div data-test-id="pierre-worker-pool">{props.children}</div>;
+  },
 }));
 
 const filesystemPanelModule = await import("./filesystem-panel");
@@ -119,6 +133,9 @@ beforeEach(() => {
   lastFilesArgs = null;
   lastFileArgs = null;
   lastTreeOptions = null;
+  pierreFileCalls.length = 0;
+  providerCalls.length = 0;
+  virtualizerCalls.length = 0;
   filesState = makeEmptyFilesState();
   fileState = makeEmptyFileState();
 });
@@ -380,6 +397,38 @@ describe("FilesystemPanel render", () => {
             && node.props["data-file-name"] === "README.md",
     );
     expect(previews).toHaveLength(1);
+    expect(providerCalls).toHaveLength(1);
+    expect(virtualizerCalls).toHaveLength(1);
+    expect(pierreFileCalls[0].file.cacheKey).toContain(
+      "fabro-sandbox-file:run_1:/README.md:",
+    );
+  });
+
+  test("renders an empty text file without mounting Pierre File", () => {
+    filesState = {
+      ...makeEmptyFilesState(),
+      data: { data: [{ name: ".dockerenv", is_dir: false, size: 0 }] },
+    };
+    fileState = {
+      ...makeEmptyFileState(),
+      data: new TextEncoder().encode("").buffer as ArrayBuffer,
+    };
+    const renderer = renderPanel();
+    act(() => {
+      lastTreeOptions?.onSelectionChange?.([".dockerenv"]);
+    });
+    expect(lastFileArgs).toEqual({
+      id:   "run_1",
+      path: "/.dockerenv",
+    });
+    expect(findByTestId(renderer, "pierre-file")).toHaveLength(0);
+    const titles = renderer.root.findAll(
+      (node) =>
+        node.type === "p" &&
+        Array.isArray(node.children) &&
+        node.children.includes("Empty file"),
+    );
+    expect(titles).toHaveLength(1);
   });
 
   test("renders binary fallback when file contents contain a null byte", () => {

apps/fabro-web/app/routes/run-sandbox/filesystem-panel.tsx

@@ -15,17 +15,23 @@ import {
 import {
   FileTree,
   useFileTree,
-  useFileTreeSelection,
 } from "@pierre/trees/react";
 import { themeToTreeStyles } from "@pierre/trees";
 import pierreDark from "@pierre/theme/pierre-dark";
-import { File } from "@pierre/diffs/react";
+import {
+  File,
+  Virtualizer,
+  WorkerPoolContextProvider,
+  type FileContents,
+} from "@pierre/diffs/react";
 import type { SandboxFileEntry } from "@qltysh/fabro-api-client";
 
 import { useSandboxFile, useSandboxFiles } from "../../lib/queries";
 import { ApiError } from "../../lib/api-client";
 import { EmptyState, ErrorState, LoadingState } from "../../components/state";
 import { SECONDARY_BUTTON_CLASS, Tooltip } from "../../components/ui";
+import { workerFactory } from "../../lib/pierre-diffs-worker";
+import { stringHash } from "../run-files/cache-keys";
 
 export const DEFAULT_DIR = "/";
 
@@ -34,6 +40,8 @@ export const DEFAULT_DIR = "/";
 // download-only state.
 export const TEXT_PREVIEW_BYTE_LIMIT = 256 * 1024;
 const BINARY_SAMPLE_BYTES = 8 * 1024;
+const pierrePoolOptions = { workerFactory };
+const pierreHighlighterOptions = { theme: "pierre-dark" };
 
 type TreeThemeStyle = CSSProperties & Record<`--${string}`, string | number>;
 
@@ -114,6 +122,18 @@ export function formatFileSize(bytes: number | undefined): string | null {
   return `${(bytes / (1024 * 1024 * 1024)).toFixed(2)} GiB`;
 }
 
+export function sandboxFileCacheKey({
+  runId,
+  path,
+  contents,
+}: {
+  runId: string;
+  path: string;
+  contents: string;
+}): string {
+  return `fabro-sandbox-file:${runId}:${path}:${stringHash(contents)}`;
+}
+
 interface BuiltTreeInputs {
   paths: string[];
   fileEntries: Map<string, SandboxFileEntry>;
@@ -486,8 +506,13 @@ function PreviewPane({
           </a>
         </div>
       </header>
-      <div className="min-h-0 flex-1 overflow-auto">
-        <PreviewBody preview={preview} fileName={name} />
+      <div className="min-h-0 flex-1">
+        <PreviewBody
+          preview={preview}
+          runId={runId}
+          fileName={name}
+          filePath={filePath}
+        />
       </div>
     </div>
   );
@@ -495,10 +520,14 @@ function PreviewPane({
 
 function PreviewBody({
   preview,
+  runId,
   fileName,
+  filePath,
 }: {
   preview: PreviewState;
+  runId: string;
   fileName: string;
+  filePath: string;
 }) {
   if (preview.status === "loading") {
     return (
@@ -538,10 +567,39 @@ function PreviewBody({
       </div>
     );
   }
+  if ((preview.text ?? "").length === 0) {
+    return (
+      <div className="flex h-full items-center justify-center p-6">
+        <EmptyState
+          title="Empty file"
+          description="This file has no contents."
+        />
+      </div>
+    );
+  }
+  const file: FileContents = {
+    name:     fileName,
+    contents: preview.text ?? "",
+    cacheKey: sandboxFileCacheKey({
+      runId,
+      path:     filePath,
+      contents: preview.text ?? "",
+    }),
+  };
   return (
-    <File
-      file={{ name: fileName, contents: preview.text ?? "" }}
-      options={{ theme: "pierre-dark", disableFileHeader: true }}
-    />
+    <WorkerPoolContextProvider
+      poolOptions={pierrePoolOptions}
+      highlighterOptions={pierreHighlighterOptions}
+    >
+      <Virtualizer
+        className="h-full min-h-0 overflow-auto"
+        contentClassName="min-w-0 pb-4"
+      >
+        <File
+          file={file}
+          options={{ theme: "pierre-dark", disableFileHeader: true }}
+        />
+      </Virtualizer>
+    </WorkerPoolContextProvider>
   );
 }