rewrite asset paths baked into the page, including JSON-escaped ones

Gitea inlines a script naming its own worker:
'\/assets\/js\/eventsource.sharedworker.…js'. That is not an attribute and its
slashes are backslash-escaped, so neither the attribute rewrite nor appSubUrl
saw it. Unprefixed, the browser resolved it against the PLATFORM's origin, got
the SPA's index.html, and reported "Uncaught SyntaxError: Unexpected token '<'"
from a file that appeared to load — along with a dead SharedWorker and a
heatmap parsing HTML as JSON.

appSubUrl covers everything Gitea's code BUILDS at runtime. This covers what is
already in the page before that code runs.

Narrow on purpose, like dev-proxy's /_bun/ rule: a blanket rewrite of absolute
paths inside JavaScript corrupts strings that are not URLs. /assets/ is Gitea's
own asset root and specific enough to match safely.

The escaping has to match the string it lands in — \/api\/gitea\/_web, not
\\/api — or the JS stops parsing. Both forms are tested against a live sample.

sidecar/rewrite.test.ts

@@ -1,5 +1,12 @@
 import { describe, expect, test } from 'bun:test';
-import { rewriteHtml, rewriteLocation, rewriteSetCookie, rewriteAttributes, rewriteAppSubUrl } from './rewrite';
+import {
+  rewriteHtml,
+  rewriteLocation,
+  rewriteSetCookie,
+  rewriteAttributes,
+  rewriteAppSubUrl,
+  rewriteAssetPaths,
+} from './rewrite';
 
 const BASE = '/api/gitea/_web';
 const ORIGINS = ['http://localhost:3000', 'https://gitea.officer.dev'];
@@ -90,6 +97,28 @@ describe('redirects — the most visible failure, because the frame just leaves'
   });
 });
 
+describe('asset paths baked into the page before Gitea\'s own code runs', () => {
+  test('the JSON-escaped form — the SharedWorker URL that broke the dashboard', () => {
+    // Verbatim from a live page. Not an attribute, and its slashes are backslash-escaped, so neither the
+    // attribute rewrite nor appSubUrl touches it. Unprefixed, the browser resolves it against the
+    // PLATFORM's origin, gets the SPA's index.html, and reports `Unexpected token '<'` from a file that
+    // looks like it loaded.
+    const real = String.raw`'\/assets\/js\/eventsource.sharedworker.DV1Z2uJv.js',`;
+    const out = rewriteAssetPaths(real, BASE);
+    expect(out).toContain(String.raw`'\/api\/gitea\/_web\/assets\/js\/eventsource`);
+    // The escaping has to match the string it lands in, or the JS stops parsing.
+    expect(out).not.toContain(String.raw`\\/api`);
+  });
+
+  test('the plain quoted form too', () => {
+    expect(rewriteAssetPaths(`x = "/assets/img/logo.svg"`, BASE)).toContain(`"${BASE}/assets/img/logo.svg"`);
+  });
+
+  test('leaves a path that is not an asset root alone', () => {
+    expect(rewriteAssetPaths(`x = "/user/login"`, BASE)).toBe(`x = "/user/login"`);
+  });
+});
+
 describe('the origin Gitea declares about itself', () => {
   test('is discovered from window.config, so ROOT_URL needs no second setting', () => {
     // The sidecar only knows where it FETCHES from — usually loopback. The public origin lives in the

sidecar/rewrite.ts

@@ -103,6 +103,29 @@ export function rewriteAbsolute(html: string, base: string, origins: string[]):
   return out;
 }
 
+/**
+ * Asset paths embedded in JavaScript, including JSON-escaped ones.
+ *
+ * Gitea inlines a script that names its own worker: `'\\/assets\\/js\\/eventsource.sharedworker.…js'`.
+ * That is not an attribute and its slashes are backslash-escaped, so neither of the rewrites above sees
+ * it. Unprefixed, the browser resolves it against the PLATFORM's origin, gets the SPA's index.html, and
+ * reports `Uncaught SyntaxError: Unexpected token '<'` from a file that looks like it loaded.
+ *
+ * NARROW on purpose, like dev-proxy's `/_bun/` rule. A blanket rewrite of absolute paths inside
+ * JavaScript corrupts strings that are not URLs; `/assets/` is Gitea's own asset root and specific
+ * enough to match safely.
+ *
+ * `appSubUrl` covers everything Gitea's code BUILDS at runtime. This covers what is already baked into
+ * the page before that code runs.
+ */
+export function rewriteAssetPaths(html: string, base: string): string {
+  // `/api/gitea/_web` becomes `\/api\/gitea\/_web`, matching the escaping of the string it is going into.
+  const escapedBase = base.split('/').join('\\/');
+  return html
+    .replace(/\\\/assets\\\//g, `${escapedBase}\\/assets\\/`)
+    .replace(/(["'`])\/assets\//g, (_m, q: string) => `${q}${base}/assets/`);
+}
+
 /**
  * The public URL Gitea believes it has, read out of its own page.
  *
@@ -128,7 +151,7 @@ export function rewriteHtml(html: string, base: string, origins: string[]): stri
   // attribute pass then skips as already-prefixed.
   const declared = appUrlOrigin(html);
   const all = declared ? [...origins, declared] : origins;
-  return rewriteAttributes(rewriteAppSubUrl(rewriteAbsolute(html, base, all), base), base);
+  return rewriteAssetPaths(rewriteAttributes(rewriteAppSubUrl(rewriteAbsolute(html, base, all), base), base), base);
 }
 
 /**