fix: use optimizeDeps.exclude for preact to avoid duplicate instances

Instead of disabling the dependency optimizer entirely (noDiscovery),
exclude only preact ecosystem packages from optimization. This allows
CJS packages like mime-db to be pre-bundled for the browser while
preventing duplicate preact instances when remote (JSR) islands
resolve deps to /@fs/ paths.

Also extends the CJS shim to work in both SSR (with createRequire)
and client (with stub require) environments.

All dev server tests pass (35/36 — 1 pre-existing flaky failure
on remote island that also fails on main). All 31 build tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bartlomieju

deno.lock

@@ -149,6 +149,26 @@ export function fresh(config?: FreshViteConfig): Plugin[] {
             },
           },
 
+          optimizeDeps: {
+            // Exclude preact ecosystem from optimizer to prevent
+            // duplicate instances with remote (JSR) islands. JSR
+            // islands resolve deps to /@fs/ paths, so if the
+            // optimizer also bundles them to /.vite/deps/, two
+            // separate instances load. Other CJS packages (like
+            // mime-db) are optimized normally.
+            exclude: [
+              "preact",
+              "preact/hooks",
+              "preact/jsx-runtime",
+              "preact/jsx-dev-runtime",
+              "preact/debug",
+              "preact/compat",
+              "@preact/signals",
+              "@preact/signals-core",
+              "preact-render-to-string",
+            ],
+          },
+
           publicDir: pathWithRoot(fConfig.staticDir[0], config.root),
 
           builder: {

packages/plugin-vite/src/mod.ts

@@ -184,10 +184,10 @@ export function deno(): Plugin {
         ? ssrLoader
         : browserLoader;
 
-      // In dev mode, non-external CJS files go through Vite's SSR
-      // module runner which evaluates them as ESM. Wrap CJS files
-      // with an ESM shim that provides module/exports/require. In
-      // build mode, Rollup's @rollup/plugin-commonjs handles CJS.
+      // In dev mode, CJS files need to be wrapped in an ESM shim:
+      // - SSR: module runner evaluates as ESM, needs module/exports/require
+      // - Client: browser evaluates as ESM, needs module/exports
+      // In build mode, Rollup's @rollup/plugin-commonjs handles CJS.
       if (
         isDev &&
         !id.startsWith("\0") &&
@@ -204,13 +204,20 @@ export function deno(): Plugin {
               code.includes("exports.") ||
               code.includes("require("))
           ) {
-            const wrapped = `
-import { createRequire as __fresh_createRequire } from "node:module";
-import { fileURLToPath as __fresh_fileURLToPath } from "node:url";
-import { dirname as __fresh_dirname } from "node:path";
-var __filename = __fresh_fileURLToPath(import.meta.url);
-var __dirname = __fresh_dirname(__filename);
-var require = __fresh_createRequire(import.meta.url);
+            const isServer =
+              this.environment.config.consumer === "server";
+            const preamble = isServer
+              ? `import { createRequire as __cjs_createRequire } from "node:module";
+import { fileURLToPath as __cjs_fileURLToPath } from "node:url";
+import { dirname as __cjs_dirname } from "node:path";
+var __filename = __cjs_fileURLToPath(import.meta.url);
+var __dirname = __cjs_dirname(__filename);
+var require = __cjs_createRequire(import.meta.url);`
+              : `var __filename = "";
+var __dirname = "";
+var require = (id) => { throw new Error("require() not supported in browser: " + id); };`;
+
+            const wrapped = `${preamble}
 var module = { exports: {} };
 var exports = module.exports;