From 3fb0fe9eae0da890dff524361bafda134d8eb82e Mon Sep 17 00:00:00 2001
From: Ben Vinegar <ben@benv.ca>
Date: Tue, 30 Jun 2026 22:34:09 -0400
Subject: [PATCH] fix(render): honor system mode for direct rich surfaces

---
 .changeset/system-surface-schemes.md |  5 ++++
 server/richRender.ts                 |  2 +-
 server/surfacePage.ts                | 36 ++++++++++++++++++----------
 test/richRender.test.ts              | 13 ++++++++++
 test/surfacePage.test.ts             | 31 ++++++++++++++++++++----
 5 files changed, 69 insertions(+), 18 deletions(-)
 create mode 100644 .changeset/system-surface-schemes.md

diff --git a/.changeset/system-surface-schemes.md b/.changeset/system-surface-schemes.md
new file mode 100644
index 0000000..330e90f
--- /dev/null
+++ b/.changeset/system-surface-schemes.md
@@ -0,0 +1,5 @@
+---
+"sideshow": patch
+---
+
+Let direct rich-surface renders without an explicit mode follow the browser's system color scheme.
diff --git a/server/richRender.ts b/server/richRender.ts
index b3615cd..6b44e4e 100644
--- a/server/richRender.ts
+++ b/server/richRender.ts
@@ -395,7 +395,7 @@ export async function renderDiff(
   const options = {
     diffStyle: part.layout ?? "unified",
     theme: { dark: shiki.dark, light: shiki.light },
-    themeType: opts.mode === "dark" ? "dark" : "light",
+    themeType: opts.mode ?? "system",
     preferredHighlighter: "shiki-js",
   } as const;
   const rendered = await Promise.all(
diff --git a/server/surfacePage.ts b/server/surfacePage.ts
index 81690a1..ce26163 100644
--- a/server/surfacePage.ts
+++ b/server/surfacePage.ts
@@ -31,12 +31,11 @@ const KIT_ACCENTS_DARK: Record<string, string> = {
 };
 const kitAccentCss = (mode?: Mode): string => schemeCss(KIT_ACCENTS_LIGHT, KIT_ACCENTS_DARK, mode);
 
-// When a scheme is pinned, force the document's used color-scheme to match so
-// the UA-painted canvas, scrollbars, and native form controls follow it too
-// (the token vars alone don't drive those). Overrides the static
-// `color-scheme: light dark` default the kit/base CSS sets. Empty when the
-// scheme is left to the OS, preserving the media-query behavior unchanged.
-const colorSchemeCss = (mode?: Mode): string => (mode ? `:root{color-scheme:${mode}}` : "");
+// Force the document's used color-scheme so the UA-painted canvas, scrollbars,
+// and native form controls follow the same scheme as the theme vars (the vars
+// alone don't drive those). Pinned frames get a single scheme; unpinned/direct
+// loads opt into both schemes so the browser can resolve the user's system mode.
+const colorSchemeCss = (mode?: Mode): string => `:root{color-scheme:${mode ?? "light dark"}}`;
 
 // Origins html surfaces may load external resources from. Mirrors the allowlist
 // agents already know from Claude's inline widget surface.
@@ -357,7 +356,8 @@ ${doc.body}
 // own DOMPurify (securityLevel 'strict') runs first; the opaque origin is the
 // second boundary. Theme colors are baked into the diagram at render time, so —
 // like shiki's flip — they're PINNED to the chrome-resolved mode the viewer
-// passed (mermaid can't do a media-query flip); absent mode defaults to light.
+// passed. On a direct no-mode load, the iframe's own JS chooses the user's
+// system scheme before mermaid renders.
 
 const MERMAID_CSS = `
 body { margin: 0; padding: 14px 16px; background: transparent; text-align: center; }
@@ -475,21 +475,33 @@ export function renderMermaidPage(doc: {
 }): string {
   const theme =
     typeof doc.theme === "string" || doc.theme == null ? themeById(doc.theme) : doc.theme;
-  const palette = doc.mode === "dark" ? theme.dark : theme.light;
-  const { themeVariables, themeCSS } = mermaidThemeVars(palette, doc.mode);
+  const enc = (v: unknown) => JSON.stringify(v).replace(/</g, "\\u003c");
+  const pinned = doc.mode
+    ? mermaidThemeVars(doc.mode === "dark" ? theme.dark : theme.light, doc.mode)
+    : null;
+  const light = pinned ? null : mermaidThemeVars(theme.light, "light");
+  const dark = pinned ? null : mermaidThemeVars(theme.dark, "dark");
+  const autoTheme = pinned
+    ? ""
+    : `const __mql = window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)');
+const __systemDark = !!(__mql && __mql.matches);
+const themeVariables = __systemDark ? ${enc(dark!.themeVariables)} : ${enc(light!.themeVariables)};
+const themeCSS = __systemDark ? ${enc(dark!.themeCSS)} : ${enc(light!.themeCSS)};`;
+  const themeConfig = pinned
+    ? `themeVariables: ${enc(pinned.themeVariables)},\n  themeCSS: ${enc(pinned.themeCSS)},`
+    : `themeVariables,\n  themeCSS,`;
   // Embed source + theme as JS literals; escape `<` so a `</script>` in the
   // diagram source can't break out of the module script.
-  const enc = (v: unknown) => JSON.stringify(v).replace(/</g, "\\u003c");
   const loader = `
 import mermaid from ${enc(MERMAID_CDN)};
 const src = ${enc(doc.mermaid ?? "")};
+${autoTheme}
 mermaid.initialize({
   startOnLoad: false,
   securityLevel: 'strict',
   suppressErrorRendering: true,
   theme: 'base',
-  themeVariables: ${enc(themeVariables)},
-  themeCSS: ${enc(themeCSS)},
+  ${themeConfig}
 });
 const el = document.getElementById('m');
 try {
diff --git a/test/richRender.test.ts b/test/richRender.test.ts
index 4b42c66..749a585 100644
--- a/test/richRender.test.ts
+++ b/test/richRender.test.ts
@@ -121,6 +121,19 @@ test("renderDiff: an empty patch and no files throws No diff content", async ()
   await assert.rejects(() => renderDiff({ kind: "diff", patch: "" }), /No diff content/);
 });
 
+test("renderDiff: absent mode keeps the diff in system color-scheme", async () => {
+  const diff: DiffSurface = {
+    kind: "diff",
+    files: [{ filename: "f.ts", before: "const x = 1", after: "const x = 2" }],
+  };
+  const { body } = await renderDiff(diff);
+  assert.match(body, /color-scheme:\s*light dark/);
+  assert.doesNotMatch(
+    body.match(/<style data-theme-css="">[\s\S]*?<\/style>/)?.[0] ?? "",
+    /color-scheme:\s*(?:light|dark);/,
+  );
+});
+
 test("renderTerminal: ANSI codes are converted and a window bar is rendered", async () => {
   const term: TerminalSurface = {
     kind: "terminal",
diff --git a/test/surfacePage.test.ts b/test/surfacePage.test.ts
index 569f8a9..3e3f7a0 100644
--- a/test/surfacePage.test.ts
+++ b/test/surfacePage.test.ts
@@ -139,11 +139,15 @@ test("a pinned mode forces color-scheme into both html parts and transparent ric
   );
   assert.ok(gh.includes("--c-teal-bg: rgba(31, 169, 150, 0.18)"), "kit teal accent pinned to dark");
 
-  // light pins the other way; absent mode keeps the OS-driven media query
+  // light pins the other way; absent mode keeps OS-driven theme vars and opts the
+  // document into both UA color schemes so the browser can resolve the system mode.
   const light = renderHtmlPage({ title: "t", html: "<p>x</p>", origin: ORIGIN, mode: "light" });
   assert.ok(/:root\{color-scheme:light\}/.test(light), "color-scheme must be pinned to light");
   const auto = renderHtmlPage({ title: "t", html: "<p>x</p>", origin: ORIGIN });
-  assert.ok(!auto.includes("color-scheme:dark"), "no mode → no forced scheme");
+  assert.ok(
+    /:root\{color-scheme:light dark\}/.test(auto),
+    "no mode → browser resolves light/dark from the OS",
+  );
   assert.ok(auto.includes("@media (prefers-color-scheme: dark)"), "no mode → OS media query kept");
 
   // rich/comment frames pin the same way, color-scheme INCLUDED. A sandboxed
@@ -164,7 +168,8 @@ test("a pinned mode forces color-scheme into both html parts and transparent ric
     rich.includes(`--text: ${dark.text}`),
     "rich frame carries the pinned dark chrome vars",
   );
-  // light pins light; an unpinned (no-mode) frame leaves the scheme to the OS.
+  // light pins light; an unpinned (no-mode) frame opts into both schemes so the
+  // browser resolves the user's system mode instead of defaulting the canvas to light.
   assert.ok(
     /:root\{color-scheme:light\}/.test(
       renderSandboxedPart({ body: "x", css: "", origin: ORIGIN, mode: "light" }),
@@ -172,8 +177,10 @@ test("a pinned mode forces color-scheme into both html parts and transparent ric
     "light mode pins color-scheme:light",
   );
   assert.ok(
-    !/:root\{color-scheme:/.test(renderSandboxedPart({ body: "x", css: "", origin: ORIGIN })),
-    "no mode → scheme left to the OS (the @media query may still mention prefers-color-scheme)",
+    /:root\{color-scheme:light dark\}/.test(
+      renderSandboxedPart({ body: "x", css: "", origin: ORIGIN }),
+    ),
+    "no mode → browser resolves light/dark from the OS",
   );
 });
 
@@ -234,6 +241,20 @@ test("a mermaid page pins mermaid's derived colors to the scheme so the whole di
   }
 });
 
+test("a no-mode mermaid page chooses the user's system scheme in the iframe", () => {
+  const auto = renderMermaidPage({ mermaid: "graph TD; A-->B", origin: ORIGIN, theme: "github" });
+  assert.ok(
+    auto.includes("matchMedia('(prefers-color-scheme: dark)')"),
+    "direct no-mode mermaid load should read the browser's system scheme",
+  );
+  assert.ok(auto.includes('"darkMode":true'), "auto loader embeds dark mermaid variables");
+  assert.ok(auto.includes('"darkMode":false'), "auto loader embeds light mermaid variables");
+  assert.ok(
+    /:root\{color-scheme:light dark\}/.test(auto),
+    "the document itself opts into system light/dark",
+  );
+});
+
 test("renderSandboxedPart embeds the body and css inside the sandbox doc", () => {
   const doc = renderSandboxedPart({
     body: "<p>hello</p>",