fix(html-preview): deliver the document through srcdoc so a sandboxed frame can load it

The preview showed nothing but the bare word `forbidden` in DSH Desktop. The
cause is not this plugin's routes but the host, one layer above them:
`DesktopWebServer` wraps EVERY route with `permits()`, and in the default
isolated/扩展 mode `ordinaryBrowserEnabled` is false, so a request is served
only when it carries the Electron renderer capability
(`x-dsh-desktop-renderer`). That header is attached solely to MAIN-frame and
SAME-ORIGIN-subframe traffic (`requestComesFromRendererOrigin`), and a frame
sandboxed WITHOUT allow-same-origin is an opaque origin at request time — so
the preview document request could never be served, while every other sidebar
route (all main-frame) worked fine.

The fix keeps the sandbox boundary exactly where it was and moves the delivery:
the document is fetched by the MAIN frame — which the host does trust — and
handed to the frame through `srcdoc`. New shared component HtmlPreviewFrame
owns that path plus the sandbox tokens for BOTH surfaces (the editor viewer and
the changes tab's op render preview, which had no escape hatch at all), so the
two can no longer drift; a failed fetch falls back to handing the frame the
route URL, leaving the route's own answer visible instead of a blank pane.

`withPreviewBase` injects `<base href="<the file's route directory>">`: a
srcdoc document has no URL of its own, so without it `./style.css` would
resolve against the GUI page. A document that declares its own base is left
alone.

Two consequences, both deliberate:
- sandboxed: the frame stays opaque, so a self-contained document previews
  completely while a multi-file one shows its markup with the relative assets
  missing (those requests are opaque-origin too — inlining them is a separate
  job);
- unlocked (the warned per-surface unlock, or the setting): the srcdoc frame
  inherits the GUI origin — which is what that escape hatch already meant — and
  there relative references resolve and load like any other same-origin
  request, so multi-file previews work in that mode.

Verified: typecheck clean; build clean; 1364 passed / 9 skipped (130 files,
+13: the pure base-rewrite spec, the delivery spec, and the updated
sandbox-contract assertions, which now pin srcdoc + no route src instead of
the old route-src shape).

src/client/HtmlPreviewFrame.tsx

@@ -0,0 +1,78 @@
+/**
+ * The one HTML-preview frame both preview surfaces render.
+ *
+ * Why this is not a plain `src` to the `/sidebar/html` route: on DSH Desktop
+ * the host answers 403 `forbidden` to every request that does not carry the
+ * Electron renderer capability (`x-dsh-desktop-renderer`), and the shell
+ * attaches that header only to MAIN-frame and SAME-ORIGIN-subframe traffic
+ * (`requestComesFromRendererOrigin`). A frame sandboxed WITHOUT
+ * allow-same-origin is an opaque origin at request time, so its document
+ * request never carries the capability and the preview shows nothing but the
+ * bare refusal.
+ *
+ * So the document is fetched by the MAIN frame — which does carry it — and
+ * handed to the frame through `srcdoc`. The `sandbox` attribute stays exactly
+ * as before: the preview still runs in an opaque origin, and the route's own
+ * workspace fence still bounds what can be fetched.
+ *
+ * Two consequences worth knowing:
+ * - sandboxed: an opaque origin cannot make network requests either, so a
+ *   self-contained document previews completely while a multi-file one shows
+ *   its markup with the relative assets missing (inlining them is a separate
+ *   job, not this component's);
+ * - unsandboxed (the warned per-surface unlock, or the setting): the srcdoc
+ *   frame inherits the GUI's origin — which is precisely what that escape
+ *   hatch already meant — and there its relative references resolve and load
+ *   like any other same-origin request.
+ *
+ * A fetch that fails hands the frame the route URL instead, so the route's
+ * own answer (its JSON error, or the refusal) stays visible rather than being
+ * replaced by a blank pane.
+ */
+import { useEffect, useState, type ReactNode } from 'react'
+import { HTML_IFRAME_SANDBOX, withPreviewBase } from './html-preview.ts'
+
+/** One preview frame. */
+export function HtmlPreviewFrame(props: {
+  /** The `/sidebar/html/<sessionId>/<path…>` route URL of the SAVED file. */
+  url: string
+  /** Whether the opaque-origin sandbox applies (the default for both surfaces). */
+  sandboxed: boolean
+  className?: string
+  title: string
+}): ReactNode {
+  const { url, sandboxed, className, title } = props
+  const [doc, setDoc] = useState<string | null>(null)
+  const [failed, setFailed] = useState(false)
+  useEffect(() => {
+    const controller = new AbortController()
+    setDoc(null)
+    setFailed(false)
+    void fetch(url, { signal: controller.signal })
+      .then(async (response) => {
+        if (!response.ok) throw new Error(String(response.status))
+        return response.text()
+      })
+      .then((text) => { setDoc(text) })
+      .catch((error: unknown) => {
+        // An abort is this effect's own cleanup, not a failure.
+        if ((error as { name?: string }).name !== 'AbortError') setFailed(true)
+      })
+    return () => { controller.abort() }
+  }, [url])
+  // The delivery path depends only on what we have: the fetched markup, the
+  // route's own answer after a failed fetch, or an empty frame while loading.
+  const delivery = doc !== null
+    ? { srcDoc: withPreviewBase(doc, url) }
+    : failed ? { src: url } : { srcDoc: '' }
+  return (
+    <iframe
+      {...delivery}
+      className={className}
+      sandbox={sandboxed ? HTML_IFRAME_SANDBOX : undefined}
+      referrerPolicy="no-referrer"
+      allow=""
+      title={title}
+    />
+  )
+}

src/client/TextEditor.tsx

@@ -36,7 +36,7 @@ import { LazyMermaidMarkdown, MarkdownDocument, type MarkdownHtmlMedia } from '.
 import { MdToc } from './md-toc.tsx'
 import { splitMermaidBlocks } from './mermaid-blocks.ts'
 import { t } from './locales.ts'
-import { HTML_IFRAME_SANDBOX } from './html-preview.ts'
+import { HtmlPreviewFrame } from './HtmlPreviewFrame.tsx'
 import type { EditorToolbarState, FileViewerProps } from './service.ts'
 import css from './sidebar.module.css'
 
@@ -545,16 +545,14 @@ export function TextEditor(props: FileViewerProps) {
             onUnlock={() => { setLocalUnlock(true) }}
             onRestore={() => { setLocalUnlock(false) }}
           />
-          {/* Route-src (never srcdoc — a srcdoc frame inherits the parent
-              origin when unsandboxed; the route URL keeps the frame
-              cross-origin by construction). The preview shows the SAVED
-              file; the draft is only visible in edit mode. */}
-          <iframe
+          {/* The preview shows the SAVED file; the draft is only visible in
+              edit mode. HtmlPreviewFrame owns both the delivery path
+              (main-frame fetch -> srcdoc, which a sandboxed frame could not
+              reach on its own) and the sandbox tokens. */}
+          <HtmlPreviewFrame
             className={css.editorHtml}
-            src={htmlUrl(scope, path)}
-            sandbox={htmlNoSandbox ? undefined : HTML_IFRAME_SANDBOX}
-            referrerPolicy="no-referrer"
-            allow=""
+            url={htmlUrl(scope, path)}
+            sandboxed={!htmlNoSandbox}
             title={path}
           />
         </>

src/client/changes/DiffPane.tsx

@@ -15,7 +15,7 @@ import { api, htmlUrl } from '../api.ts'
 import { t } from '../locales.ts'
 import { baseName } from '../paths.ts'
 import { resolveSidebarPath } from '../produced-files.ts'
-import { HTML_IFRAME_SANDBOX } from '../html-preview.ts'
+import { HtmlPreviewFrame } from '../HtmlPreviewFrame.tsx'
 import type { SidebarDiffRef, SidebarTab } from '../state.ts'
 import { DiffRows, ReadRows } from '../diff/DiffRows.tsx'
 import { PdfView } from '../PdfView.tsx'
@@ -66,19 +66,18 @@ function diffOf(op: FileOp, prior: string | undefined): readonly DiffRow[] {
   return []
 }
 
-/** The render view of one html op target: the route-src iframe. Extracted
- *  (and exported) so the always-sandboxed contract is pinned directly by the
- *  sandbox spec — this surface has NO no-sandbox escape hatch. */
+/** The render view of one html op target. Extracted (and exported) so the
+ *  always-sandboxed contract is pinned directly by the sandbox spec — this
+ *  surface has NO no-sandbox escape hatch, so it always takes the srcdoc
+ *  delivery path. */
 export function HtmlRenderPreview(props: { src: string; title: string }) {
   return (
     <div className={css.htmlPane}>
-      <iframe
+      <HtmlPreviewFrame
         className={css.htmlFrame}
+        url={props.src}
+        sandboxed
         title={props.title}
-        src={props.src}
-        sandbox={HTML_IFRAME_SANDBOX}
-        referrerPolicy="no-referrer"
-        allow=""
       />
     </div>
   )

src/client/html-preview.ts

@@ -1,7 +1,8 @@
 /**
  * Shared vocabulary of the sidebar's HTML preview surfaces (the editor's
  * html viewer and the changes tab's op render preview): the sandbox tokens
- * both preview iframes load with.
+ * both preview iframes load with, plus the document-base rewrite the srcdoc
+ * delivery path needs.
  */
 
 /**
@@ -13,3 +14,47 @@
  * render preview always stays sandboxed.
  */
 export const HTML_IFRAME_SANDBOX = 'allow-scripts allow-popups allow-downloads allow-modals'
+
+/** Whether a document already declares its own base URL. */
+const HAS_BASE = /<base[\s/>]/i
+
+/** Index just past the element a `<base>` must precede to land in <head>. */
+function insertionPoint(html: string): number {
+  const head = /<head\b[^>]*>/i.exec(html)
+  if (head !== null) return head.index + head[0].length
+  const htmlTag = /<html\b[^>]*>/i.exec(html)
+  if (htmlTag !== null) return htmlTag.index + htmlTag[0].length
+  return 0
+}
+
+/**
+ * The directory part of a preview route URL, with its trailing slash.
+ * @param url - the `/sidebar/html/<sessionId>/<path…>` route URL.
+ * @returns the URL up to and including the last `/`.
+ */
+export function previewBaseOf(url: string): string {
+  const cut = url.lastIndexOf('/')
+  return cut < 0 ? url : url.slice(0, cut + 1)
+}
+
+/**
+ * Point a preview document's relative references back at its own route.
+ *
+ * A `srcdoc` document has no URL of its own — the browser resolves its
+ * relative references against the EMBEDDING page's URL, so `./style.css` in a
+ * previewed file would resolve to the GUI's origin and hit whatever route
+ * happens to match. Injecting `<base href="<the file's route directory>">`
+ * keeps relative resolution inside `/sidebar/html/…`, where the file's own
+ * siblings live. A document that declares its own `<base>` is left alone: the
+ * author said what the base is.
+ *
+ * @param html - the previewed document's markup.
+ * @param url - the route URL the document was fetched from.
+ * @returns the markup, with a base element injected when it had none.
+ */
+export function withPreviewBase(html: string, url: string): string {
+  if (HAS_BASE.test(html)) return html
+  const href = previewBaseOf(url).replace(/"/g, '&quot;')
+  const at = insertionPoint(html)
+  return `${html.slice(0, at)}<base href="${href}">${html.slice(at)}`
+}