fix: inject client entry on islands-free pages for HMR (#3810)

Fixes #3806

---------

Co-authored-by: fibibot <fibibot@users.noreply.github.com>
Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>

Author: fibibot

packages/fresh/src/build_cache.ts

@@ -17,6 +17,12 @@ export interface FileSnapshot {
 export interface BuildSnapshot<State> {
   version: string;
   clientEntry: string;
+  /**
+   * When defined, forces the boot script to be emitted in dev so HMR
+   * listeners attach even on island-free pages. The value itself is
+   * currently unused — only its presence matters. Undefined outside of dev.
+   */
+  hmrClientEntry?: string;
   fsRoutes: FsRouteFile<State>[];
   staticFiles: Map<string, FileSnapshot>;
   islands: ServerIslandRegistry;
@@ -51,13 +57,15 @@ export class ProdBuildCache<State> implements BuildCache<State> {
   #snapshot: BuildSnapshot<State>;
   islandRegistry: ServerIslandRegistry;
   clientEntry: string;
+  hmrClientEntry: string | undefined;
   features = { errorOverlay: false };
 
   constructor(public root: string, snapshot: BuildSnapshot<State>) {
     setBuildId(snapshot.version);
     this.#snapshot = snapshot;
     this.islandRegistry = snapshot.islands;
     this.clientEntry = snapshot.clientEntry;
+    this.hmrClientEntry = snapshot.hmrClientEntry;
   }
 
   getEntryAssets(): string[] {

packages/fresh/src/dev/dev_build_cache.ts

@@ -458,6 +458,7 @@ export async function generateSnapshotServer(
     outDir: string;
     buildId: string;
     clientEntry: string;
+    hmrClientEntry?: string;
     islands: IslandModChunk[];
     // deno-lint-ignore no-explicit-any
     fsRoutesFiles: FsRouteFileNoMod<any>[];
@@ -524,12 +525,17 @@ export async function generateSnapshotServer(
   const entryAssets = options.entryAssets.map((url) => JSON.stringify(url))
     .join(",\n");
 
+  const hmrClientEntryDecl = options.hmrClientEntry !== undefined
+    ? `export const hmrClientEntry = ${JSON.stringify(options.hmrClientEntry)}`
+    : "";
+
   return `${EDIT_WARNING}
 import { IslandPreparer } from "fresh/internal";
 ${islandImports}
 ${fsRouteImports}
 
 export const clientEntry = ${JSON.stringify(options.clientEntry)}
+${hmrClientEntryDecl}
 export const version = ${JSON.stringify(options.buildId)}
 
 export const islands = new Map();

packages/plugin-vite/src/plugins/server_snapshot.ts

@@ -165,10 +165,19 @@ export function serverSnapshot(options: ResolvedFreshViteConfig): Plugin[] {
           const staticFiles: PendingStaticFile[] = [];
           let islandMods: IslandModChunk[] = [];
           let clientEntry = "/@id/fresh:client-entry";
+          let hmrClientEntry: string | undefined;
           let buildId = "";
           const entryAssets: string[] = [];
 
           if (isDev && server !== undefined) {
+            // Set hmrClientEntry so the SSR runtime always emits a boot script
+            // in dev, even when a page has zero islands. Without this, edits
+            // to island-free routes never trigger a browser reload because the
+            // `fresh:reload` WebSocket listener is never attached. The value
+            // is used as a marker only — its presence is what matters, not
+            // what it points to.
+            hmrClientEntry = clientEntry;
+
             for (const id of islands.keys()) {
               const mod = server.environments.client.moduleGraph.getModuleById(
                 id,
@@ -380,6 +389,7 @@ export function serverSnapshot(options: ResolvedFreshViteConfig): Plugin[] {
             staticFiles,
             buildId,
             clientEntry,
+            hmrClientEntry,
             entryAssets,
             fsRoutesFiles: result.routes,
             islands: islandMods,

packages/plugin-vite/tests/dev_server_test.ts

@@ -93,6 +93,23 @@ integrationTest("vite dev - starts without islands/ dir", async () => {
   });
 });
 
+// Issue: https://github.com/denoland/fresh/issues/3806
+// Pages without islands must still load the client entry in dev so the
+// HMR `fresh:reload` listener attaches and route edits trigger a refresh.
+integrationTest(
+  "vite dev - injects client entry on islands-free pages for HMR",
+  async () => {
+    const fixture = path.join(FIXTURE_DIR, "no_islands");
+    await withDevServer(fixture, async (address) => {
+      const res = await fetch(`${address}/`);
+      const text = await res.text();
+      expect(text).toContain("ok");
+      expect(text).toContain("/@id/fresh:client-entry");
+      expect(text).toMatch(/import\s*\{\s*boot\s*\}/);
+    });
+  },
+);
+
 integrationTest("vite dev - starts without routes/ dir", async () => {
   const fixture = path.join(FIXTURE_DIR, "no_routes");
   await withDevServer(fixture, async (address) => {