Push HMR updates before the SSR rebuild in dev

Client-only edits are sent to the browser right after the hydration build,
without re-running server functions; SSR rebuilds afterwards and props are
re-fetched only when a .server.ts output changed or after fullRebuild. Page
requests wait for an in-flight SSR rebuild. The SSR build no longer runs
Tailwind, the PostCSS processor is reused across rebuilds, and the watcher
debounce drops from 200ms to 50ms.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/__tests__/functions/server/server-post-build-fn.test.ts

@@ -3,7 +3,9 @@ import { describe, expect, test, beforeEach, afterEach } from "bun:test";
 // NOTE: this file runs the REAL grabPageComponent (no module mock). The
 // callers use `return_server_res_only` + `no_retry`, so no bundling or
 // rebuilds happen as long as the fake router + artifact map are set up.
-import serverPostBuildFn from "../../../functions/server/server-post-build-fn";
+import serverPostBuildFn, {
+    pushClientUpdate,
+} from "../../../functions/server/server-post-build-fn";
 
 function makeController(page_url: string, target_map?: { local_path: string }) {
     const enqueued: string[] = [];
@@ -146,4 +148,53 @@ describe("server-post-build-fn", () => {
         expect(changed.enqueued.length).toBe(1);
         expect(untouched.enqueued.length).toBe(0);
     });
-});
+
+    describe("pushClientUpdate", () => {
+        test("pushes the new bundle without page_props to changed pages only", () => {
+            const changed = makeController("http://localhost/about", {
+                local_path: "pages/about.tsx",
+            });
+            const untouched = makeController("http://localhost/missing", {
+                local_path: "pages/404.tsx",
+            });
+            global.BUNEXT_HMR_CONTROLLERS = [changed as any, untouched as any];
+
+            pushClientUpdate(["pages/about.tsx"]);
+
+            expect(changed.enqueued.length).toBe(1);
+            const data = JSON.parse(
+                changed.enqueued[0]!.replace(/^event: update\ndata: /, ""),
+            );
+            expect(data.target_map.path).toBe("pages/about.js");
+            expect(data.page_props).toBeUndefined();
+            expect(data.reload).toBeUndefined();
+            expect(untouched.enqueued.length).toBe(0);
+        });
+
+        test("reloads a changed page that has no bundled artifact", () => {
+            const c = makeController("http://localhost/new-page", {
+                local_path: "pages/new-page.tsx",
+            });
+            global.BUNEXT_HMR_CONTROLLERS = [c as any];
+
+            pushClientUpdate(["pages/new-page.tsx"]);
+
+            expect(c.enqueued.length).toBe(1);
+            expect(c.enqueued[0]).toContain('"reload":true');
+        });
+
+        test("drops controllers whose stream is closed", () => {
+            const c = makeController("http://localhost/about", {
+                local_path: "pages/about.tsx",
+            });
+            c.controller.enqueue = () => {
+                throw new Error("closed");
+            };
+            global.BUNEXT_HMR_CONTROLLERS = [c as any];
+
+            pushClientUpdate(["pages/about.tsx"]);
+
+            expect(global.BUNEXT_HMR_CONTROLLERS.length).toBe(0);
+        });
+    });
+});