perf: parallelize mobile workspace fetch (Closes #1116)

src/frontend_bundle.py

@@ -114,7 +114,13 @@ def build_frontend(frontend_dir: Path) -> FrontendBuild:
         f'<meta name="stackchain-feature-{name}" content="{bundle.runtime_name}">'
         for name, bundle in feature_bundles.items()
     )
-    dashboard_html = dashboard_html.replace("</head>", feature_metadata + "\n</head>")
+    workspace_preload = (
+        f'<link rel="preload" as="script" '
+        f'href="{feature_bundles["today-timer"].runtime_name}">'
+    )
+    dashboard_html = dashboard_html.replace(
+        "</head>", feature_metadata + "\n" + workspace_preload + "\n</head>"
+    )
     dashboard_html = dashboard_html.replace(
         "</body>",
         f'<script src="{core.runtime_name}"></script>\n</body>',

tests/e2e/test_mobile_home_bootstrap_release.py

@@ -29,6 +29,7 @@ def test_release_artifact_bootstraps_mobile_home_and_returns_from_insights(
     browser_errors: list[str] = []
     failed_responses: list[str] = []
     workspace_requests: list[str] = []
+    launch_transfer_events: list[str] = []
     live_requests: list[str] = []
 
     try:
@@ -38,6 +39,18 @@ def test_release_artifact_bootstraps_mobile_home_and_returns_from_insights(
                 viewport={"width": width, "height": height}, ignore_https_errors=True
             )
             page = context.new_page()
+            cdp = context.new_cdp_session(page)
+            cdp.send("Network.enable")
+            cdp.send(
+                "Network.emulateNetworkConditions",
+                {
+                    "offline": False,
+                    "latency": 100,
+                    "downloadThroughput": 48 * 1024,
+                    "uploadThroughput": 48 * 1024,
+                    "connectionType": "cellular3g",
+                },
+            )
             page.on("pageerror", lambda error: browser_errors.append(error.stack or str(error)))
             page.on(
                 "console",
@@ -53,9 +66,16 @@ def test_release_artifact_bootstraps_mobile_home_and_returns_from_insights(
             )
             page.on(
                 "request",
-                lambda request: workspace_requests.append(request.url)
-                if "feature-today-timer-" in request.url
-                else None,
+                lambda request: (
+                    workspace_requests.append(request.url),
+                    launch_transfer_events.append("workspace-requested"),
+                )
+                if "feature-today-timer-" in request.url else None,
+            )
+            page.on(
+                "requestfinished",
+                lambda request: launch_transfer_events.append("core-finished")
+                if "/runtime-" in request.url else None,
             )
             page.on(
                 "request",
@@ -69,6 +89,9 @@ def test_release_artifact_bootstraps_mobile_home_and_returns_from_insights(
             page.locator('input[name="access_token"]').fill(ACCESS_TOKEN)
             page.locator("#submit-sign-in").click()
             page.wait_for_url(origin + "/", wait_until="networkidle")
+            assert launch_transfer_events.index("workspace-requested") < (
+                launch_transfer_events.index("core-finished")
+            ), launch_transfer_events
 
             expect(page.locator("#my-work-status")).to_contain_text("2")
             initial_live_requests = len(live_requests)

tests/test_frontend_bundle.py

@@ -15,6 +15,7 @@ from src.views import dashboard, feature_bundle, runtime_bundle, service_worker
 
 FRONTEND = Path(__file__).resolve().parents[1] / "frontend"
 PAGE_SCRIPT = re.compile(r'<script src="(static/[^"]+\.js)"></script>')
+SCRIPT_PRELOAD = re.compile(r'<link rel="preload" as="script" href="([^"]+)">')
 
 
 def test_page_runtime_is_one_deterministic_content_addressed_bundle(tmp_path):
@@ -119,6 +120,19 @@ def test_product_workflows_are_stable_lazy_feature_chunks(tmp_path):
     assert security_changed.feature_bundles["security-center"].runtime_name != security_center.runtime_name
 
 
+def test_mandatory_workspace_fetch_is_preloaded_without_blocking_launch():
+    build = build_frontend(FRONTEND)
+    workspace = build.feature_bundles["today-timer"]
+
+    assert SCRIPT_PRELOAD.findall(build.dashboard_html) == [workspace.runtime_name]
+    assert build.dashboard_html.count(f'<script src="{workspace.runtime_name}"></script>') == 0
+    assert len(build.runtime_gzip_bytes) <= 100 * 1024
+    assert len(workspace.runtime_gzip_bytes) <= 110 * 1024
+
+    shell_block = build.service_worker_source.split("const OPTIONAL_FEATURES = [", 1)[0]
+    assert f"BASE + '{workspace.runtime_name}'" not in shell_block
+
+
 def test_shipped_browser_bundles_are_valid_javascript(tmp_path):
     build = build_frontend(FRONTEND)
     bundles = {"core.js": build.runtime_bytes}