perf(web): PDF previews reload without starting a new pdf.js worker

apps/web/src/components/files/AttachmentFilePreview.browser.test.tsx

@@ -2522,6 +2522,7 @@ it("keeps workspace PDF scroll and zoom after reloads", async () => {
   const { preview, urls } = fixture;
   await using screen = await render(preview(urls.original, "one"));
   await expect.element(screen.getByText("1 / 3 pages", { exact: true })).toBeVisible();
+  using workers = vi.spyOn(window, "Worker");
   await screen.getByRole("button", { name: "Next page", exact: true }).click();
   await vi.waitFor(() => expect(screen.container.textContent).toContain("2 / 3 pages"));
   await screen.getByRole("button", { name: "Next page", exact: true }).click();
@@ -2576,6 +2577,7 @@ it("keeps workspace PDF scroll and zoom after reloads", async () => {
     expect(region().scrollTop).toBe(0);
     expect(region().scrollLeft).toBe(0);
   });
+  expect(workers).not.toHaveBeenCalled();
 });
 
 it("clamps workspace PDF scroll when pages shrink or disappear", async () => {

apps/web/src/components/files/pdfDocument.ts

@@ -1,4 +1,4 @@
-import { getDocument, GlobalWorkerOptions } from "pdfjs-dist";
+import { getDocument, GlobalWorkerOptions, PDFWorker } from "pdfjs-dist";
 import workerUrl from "pdfjs-dist/build/pdf.worker.min.mjs?url";
 import fallbackUrl from "pdfjs-dist/wasm/openjpeg_nowasm_fallback.js?url";
 
@@ -31,8 +31,13 @@ export class BinaryDataFactory {
   }
 }
 
+// Without a shared worker, pdf.js starts a worker and recompiles its 1.3 MB script for every document.
+let worker: PDFWorker | undefined;
+
 export function openPdf(bytes: ArrayBuffer) {
+  worker ??= new PDFWorker();
   return getDocument({
+    worker,
     data: new Uint8Array(bytes),
     useSystemFonts: true,
     useWorkerFetch: false,