diff --git a/packages/cli/src/build.ts b/packages/cli/src/build.ts
index d441f0df..c293958b 100644
--- a/packages/cli/src/build.ts
+++ b/packages/cli/src/build.ts
@@ -23,7 +23,7 @@ import {
 } from "node:fs";
 import { basename, dirname, extname, join, relative, resolve } from "node:path";
 import { buildRouter, type Route } from "@wrnexus/router";
-import { getReactiveRuntime } from "@wrnexus/csr";
+import { getComponentControllerRuntime, getReactiveRuntime } from "@wrnexus/csr";
 import {
   analyzeRuntimeImports,
   analyzeRuntimeRequirements,
@@ -109,6 +109,7 @@ export async function runBuild(appRoot: string): Promise<void> {
   const compiledDir = join(distDir, "compiled");
   const clientModulesDir = join(distDir, "client");
   const reactivePath = join(distDir, "reactive.js");
+  const controllersPath = join(distDir, "controllers.js");
   const publicDir = join(root, "public");
   const distPublicDir = join(distDir, "public");
   const config = await loadAppConfig(root);
@@ -477,6 +478,13 @@ export async function runBuild(appRoot: string): Promise<void> {
   );
   assetHash.update(reactiveCode);
   console.log(`✓ Runtime: ${reactivePath}`);
+  const controllerCode = await buildBrowserRuntime(
+    getComponentControllerRuntime(),
+    controllersPath,
+    join(compiledDir, "controllers.entry.js"),
+  );
+  assetHash.update(controllerCode);
+  console.log(`✓ Controllers: ${controllersPath}`);
 
   // 1a) Theme tokens + client switcher (always emitted; built-in light/dark).
   const theme = resolveThemeConfig(config.theme);
@@ -693,6 +701,7 @@ await createProductionServer(
   },
   {
     reactivePath: join(import.meta.dir, "reactive.js"),
+    controllersPath: join(import.meta.dir, "controllers.js"),
     clientModulesDir: join(import.meta.dir, "client"),
     themePath: join(import.meta.dir, "theme.css"),
     themeJsPath: join(import.meta.dir, "theme.js"),
diff --git a/packages/csr/src/index.ts b/packages/csr/src/index.ts
index 965aad54..34a4341a 100644
--- a/packages/csr/src/index.ts
+++ b/packages/csr/src/index.ts
@@ -17,12 +17,14 @@ export { NAV_RUNTIME } from "./nav-runtime.ts";
 export { REALTIME_RUNTIME } from "./realtime-runtime.ts";
 export { ACTION_RUNTIME } from "./action-runtime.ts";
 
-/** The reactive runtime served at `/__wrnexus/reactive.js` (plain browser JS). */
-export function getReactiveRuntime(development = false): string {
+const CONTROLLER_SECTIONS = ["PRIMARY", "UI", "PIN"] as const;
+
+function runtimeForMode(development: boolean): string {
   if (development) {
-    return REACTIVE_RUNTIME
-      .replace(/\/\*__WRNEXUS_DEV_START__\*\//g, "")
-      .replace(/\/\*__WRNEXUS_DEV_END__\*\//g, "");
+    return REACTIVE_RUNTIME.replace(/\/\*__WRNEXUS_DEV_START__\*\//g, "").replace(
+      /\/\*__WRNEXUS_DEV_END__\*\//g,
+      "",
+    );
   }
   return REACTIVE_RUNTIME.replace(
     /\/\*__WRNEXUS_DEV_START__\*\/[\s\S]*?\/\*__WRNEXUS_DEV_END__\*\//g,
@@ -30,6 +32,118 @@ export function getReactiveRuntime(development = false): string {
   );
 }
 
+function controllerSection(source: string, name: (typeof CONTROLLER_SECTIONS)[number]): string {
+  const pattern = new RegExp(
+    `/\\*__WRNEXUS_CONTROLLERS_${name}_START__\\*/([\\s\\S]*?)/\\*__WRNEXUS_CONTROLLERS_${name}_END__\\*/`,
+  );
+  const match = pattern.exec(source);
+  if (!match?.[1]) throw new Error(`Missing reactive runtime controller section: ${name}`);
+  return match[1];
+}
+
+function stripControllerSections(source: string): string {
+  for (const name of CONTROLLER_SECTIONS) {
+    source = source.replace(
+      new RegExp(
+        `/\\*__WRNEXUS_CONTROLLERS_${name}_START__\\*/[\\s\\S]*?/\\*__WRNEXUS_CONTROLLERS_${name}_END__\\*/`,
+      ),
+      "",
+    );
+  }
+  return source;
+}
+
+const CONTROLLER_LOADER = String.raw`
+  var componentControllerSelector = "[data-wrn-anchored],[data-wrn-dialog],[data-wrn-roving],[data-wrn-scrollspy],[data-wrn-splitter],[data-wrn-navbar],[data-wrn-preferences],[data-wrn-select],[data-wrn-pin-input]";
+  var componentControllerPromise = null;
+  var componentControllerUrl = (document.currentScript && document.currentScript.src)
+    ? new URL("./controllers.js", document.currentScript.src).href
+    : "/__wrnexus/controllers.js";
+  function needsComponentControllers(root) {
+    var host = root || document;
+    return !!((host.matches && host.matches(componentControllerSelector)) ||
+      (host.querySelector && host.querySelector(componentControllerSelector)));
+  }
+  function loadComponentControllers(root) {
+    if (!needsComponentControllers(root)) return;
+    if (window.__wrnexusComponentControllers) {
+      window.__wrnexusComponentControllers.hydrate(root || document);
+      return;
+    }
+    if (!componentControllerPromise) {
+      componentControllerPromise = new Promise(function (resolve, reject) {
+        var script = document.createElement("script");
+        script.src = componentControllerUrl;
+        script.defer = true;
+        script.onload = resolve;
+        script.onerror = reject;
+        document.head.appendChild(script);
+      });
+    }
+    componentControllerPromise.then(function () {
+      if (window.__wrnexusComponentControllers) window.__wrnexusComponentControllers.hydrate(root || document);
+    }).catch(function (error) { console.error("[wrnexus] failed to load component controllers", error); });
+  }
+`;
+
+/** The reactive runtime served at `/__wrnexus/reactive.js` (plain browser JS). */
+export function getReactiveRuntime(development = false): string {
+  let source = stripControllerSections(runtimeForMode(development));
+  source = source.replace("  function hydrateScopes(root) {", `${CONTROLLER_LOADER}\n  function hydrateScopes(root) {`);
+  source = source.replace(
+    "    hydrateNavbarControllers(host);\n    hydratePreferenceControllers(host);\n    hydrateSelectControllers(host);\n    hydratePinInputControllers(host);",
+    "    loadComponentControllers(host);",
+  );
+  source = source.replace("  startDocumentWatch();\n", "");
+  source = source.replace(
+    "  setupAnchoredOverlays();\n  setupModalDialogs();\n  setupRovingFocus();\n  setupScrollspy();\n  setupSplitters();",
+    "  loadComponentControllers(document);",
+  );
+  source = source.replace(
+    "  window.__wrnexusRepositionAnchored = repositionAnchored;",
+    "  window.__wrnexusRepositionAnchored = function (root) { if (window.__wrnexusComponentControllers) window.__wrnexusComponentControllers.reposition(root); };",
+  );
+  source = source.replace(
+    "  window.__wrnexusMountClientRoots = mountClientRoots;",
+    `  window.__wrnexusControllerBridge = { invokeComponentOutput: invokeComponentOutput, callServerFunction: callServerFunction, dispatchComponentEvent: dispatchComponentEvent, emitPinInputEvent: emitPinInputEvent, parseScopeDecl: parseScopeDecl, warn: ${development ? "warnOnce" : "function () {}"} };\n  window.__wrnexusMountClientRoots = mountClientRoots;`,
+  );
+  return source;
+}
+
+/** Component-specific controllers, loaded only when their marker is present. */
+export function getComponentControllerRuntime(development = false): string {
+  const source = runtimeForMode(development);
+  const sections = CONTROLLER_SECTIONS.map((name) => controllerSection(source, name)).join("\n");
+  return `
+(function () {
+  "use strict";
+  var bridge = window.__wrnexusControllerBridge || {};
+  var invokeComponentOutput = bridge.invokeComponentOutput;
+  var callServerFunction = bridge.callServerFunction;
+  var dispatchComponentEvent = bridge.dispatchComponentEvent;
+  var emitPinInputEvent = bridge.emitPinInputEvent;
+  var parseScopeDecl = bridge.parseScopeDecl;
+  var warnOnce = bridge.warn || function () {};
+${sections}
+  function hydrate(root) {
+    var host = root || document;
+    hydrateNavbarControllers(host);
+    hydratePreferenceControllers(host);
+    hydrateSelectControllers(host);
+    hydratePinInputControllers(host);
+    syncRovingGroups(host);
+  }
+  setupAnchoredOverlays();
+  setupModalDialogs();
+  setupRovingFocus();
+  setupScrollspy();
+  setupSplitters();
+  window.__wrnexusComponentControllers = { hydrate: hydrate, reposition: repositionAnchored };
+  hydrate(document);
+})();
+`.trim();
+}
+
 /** The client-side navigation runtime served at `/__wrnexus/nav.js`. */
 export function getNavRuntime(): string {
   return NAV_RUNTIME;
diff --git a/packages/csr/src/reactive-runtime.ts b/packages/csr/src/reactive-runtime.ts
index b13ef6f4..c9104d87 100644
--- a/packages/csr/src/reactive-runtime.ts
+++ b/packages/csr/src/reactive-runtime.ts
@@ -2864,6 +2864,7 @@ export const REACTIVE_RUNTIME = String.raw`
    * cleared, so repositioning is idempotent -- re-running it on a panel that
    * is already on screen changes nothing and cannot drift.
    */
+  /*__WRNEXUS_CONTROLLERS_PRIMARY_START__*/
   var ANCHORED_SELECTOR = "[data-wrn-anchored]";
   var ANCHOR_MARGIN = 8;
   var anchoredScheduled = false;
@@ -3480,6 +3481,8 @@ export const REACTIVE_RUNTIME = String.raw`
     document.addEventListener("pointercancel", endDrag);
   }
 
+  /*__WRNEXUS_CONTROLLERS_PRIMARY_END__*/
+
   function hydrateScopes(root) {
     var host = root || document;
 
@@ -3510,6 +3513,7 @@ export const REACTIVE_RUNTIME = String.raw`
     /*__WRNEXUS_DEV_END__*/
   }
 
+  /*__WRNEXUS_CONTROLLERS_UI_START__*/
   var navbarOutsideClickBound = false;
   var preferenceOutsideClickBound = false;
 
@@ -4028,6 +4032,8 @@ export const REACTIVE_RUNTIME = String.raw`
     }
   }
 
+  /*__WRNEXUS_CONTROLLERS_UI_END__*/
+
   function invokeComponentOutput(root, name, payload) {
     if (!root || !name) return undefined;
     var registry = root.__wrnexusOutputHandlers;
@@ -4135,6 +4141,7 @@ export const REACTIVE_RUNTIME = String.raw`
     }
   }
 
+  /*__WRNEXUS_CONTROLLERS_PIN_START__*/
   function setupPinInputController(root) {
     if (!root || root.__wrnexusPinInputController) return;
     var cells = Array.prototype.slice.call(root.querySelectorAll("[data-pin-cell]"));
@@ -4310,6 +4317,8 @@ export const REACTIVE_RUNTIME = String.raw`
     inputs.forEach(setupPinInputController);
   }
 
+  /*__WRNEXUS_CONTROLLERS_PIN_END__*/
+
   function parseScopeDecl(decl) {
     var initial = {};
     splitTopLevel(decl, ",").forEach(function (part) {
diff --git a/packages/csr/test/reactive.test.ts b/packages/csr/test/reactive.test.ts
index cc32796b..9a2d089a 100644
--- a/packages/csr/test/reactive.test.ts
+++ b/packages/csr/test/reactive.test.ts
@@ -1,11 +1,11 @@
 import { test, expect, beforeEach } from "bun:test";
 import { Window } from "happy-dom";
 import { REACTIVE_RUNTIME } from "../src/reactive-runtime.ts";
-import { getReactiveRuntime } from "../src/index.ts";
+import { getComponentControllerRuntime, getReactiveRuntime } from "../src/index.ts";
 import { mountHtml } from "@wrnexus/test";
 
 // Fresh DOM per test, with the runtime's globals bound.
-function mount(html: string): Window {
+function mount(html: string, runtime = REACTIVE_RUNTIME, controllers = ""): Window {
   const win = new Window() as unknown as Window & Record<string, unknown>;
   win.document.body.innerHTML = `<div id="app">${html}</div>`;
   (globalThis as Record<string, unknown>).window = win;
@@ -25,7 +25,8 @@ function mount(html: string): Window {
   (globalThis as Record<string, unknown>).CustomEvent = (
     win as unknown as { CustomEvent: unknown }
   ).CustomEvent;
-  (0, eval)(REACTIVE_RUNTIME);
+  (0, eval)(runtime);
+  if (controllers) (0, eval)(controllers);
   // Hydrate deterministically (auto-init waits on DOMContentLoaded, which the
   // test window may not fire). setupScope is idempotent, so this is safe.
   const w = win as unknown as { __wrnexusHydrateScopes?: (root: unknown) => void };
@@ -41,6 +42,28 @@ beforeEach(() => {
   delete (globalThis as Record<string, unknown>).MutationObserver;
 });
 
+test("split runtime hydrates a controller only from the controller asset", () => {
+  const core = getReactiveRuntime(true);
+  const controllers = getComponentControllerRuntime(true);
+  expect(core).not.toContain("function setupRovingFocus");
+  expect(controllers).toContain("function setupRovingFocus");
+
+  const win = mount(
+    `<div data-wrn-roving="horizontal"><button data-wrn-roving-item>One</button><button data-wrn-roving-item>Two</button></div>`,
+    core,
+    controllers,
+  );
+  const buttons = win.document.querySelectorAll("button");
+  (buttons[0] as unknown as HTMLButtonElement).focus();
+  buttons[0]!.dispatchEvent(new win.KeyboardEvent("keydown", { key: "ArrowRight", bubbles: true }));
+  expect(win.document.activeElement).toBe(buttons[1]);
+});
+
+test("a page without controller markers does not request the controller asset", () => {
+  const win = mount(`<main data-scope="count: 1"><span>{count}</span></main>`, getReactiveRuntime(true));
+  expect(win.document.querySelector('script[src$="controllers.js"]')).toBeNull();
+});
+
 test("hydrates {expr} mustaches from data-scope", () => {
   const win = mount(`<div data-scope="count: 0"><span>{count}, {count * 2}</span></div>`);
   expect(win.document.querySelector("span")!.textContent).toBe("0, 0");
diff --git a/packages/dev-server/src/assets.ts b/packages/dev-server/src/assets.ts
index 0b23a33a..6967e5ba 100644
--- a/packages/dev-server/src/assets.ts
+++ b/packages/dev-server/src/assets.ts
@@ -12,6 +12,7 @@
 
 import {
   getActionRuntime,
+  getComponentControllerRuntime,
   getReactiveRuntime,
   getNavRuntime,
   getRealtimeRuntime,
@@ -94,6 +95,8 @@ export function createDevAssetServer(
         return serveWrnBrowserArtifact(pathname) ?? new Response("Not Found", { status: 404 });
       }
       if (pathname === "/__wrnexus/reactive.js") return jsResponse(getReactiveRuntime(true));
+      if (pathname === "/__wrnexus/controllers.js")
+        return jsResponse(getComponentControllerRuntime(true));
       if (pathname === "/__wrnexus/nav.js") return jsResponse(getNavRuntime());
       if (pathname === "/__wrnexus/realtime.js") return jsResponse(getRealtimeRuntime());
       if (pathname === "/__wrnexus/actions.js") return jsResponse(getActionRuntime());
diff --git a/packages/dev-server/src/prod.ts b/packages/dev-server/src/prod.ts
index 4f7fdd97..237598a5 100644
--- a/packages/dev-server/src/prod.ts
+++ b/packages/dev-server/src/prod.ts
@@ -20,6 +20,7 @@ import {
 } from "@wrnexus/router";
 import {
   getActionRuntime,
+  getComponentControllerRuntime,
   getReactiveRuntime,
   getNavRuntime,
   getRealtimeRuntime,
@@ -92,6 +93,8 @@ export interface ProdOptions {
   stylesIncludeFramework?: boolean;
   /** Absolute path to the pre-built reactive runtime. */
   reactivePath?: string;
+  /** Absolute path to the on-demand component controller runtime. */
+  controllersPath?: string;
   /** Absolute directory containing bundled per-WRN browser modules. */
   clientModulesDir?: string;
   /** Absolute path to the pre-built theme stylesheet (`theme.css`). */
@@ -341,6 +344,10 @@ function createProdAssetServer(opts: ProdOptions): AssetServer {
         }
         return new Response(getReactiveRuntime(), { headers: JS_HEADERS });
       }
+      if (pathname === "/__wrnexus/controllers.js") {
+        if (opts.controllersPath) return serveFile(opts.controllersPath, JS_HEADERS);
+        return new Response(getComponentControllerRuntime(), { headers: JS_HEADERS });
+      }
       if (pathname === "/__wrnexus/nav.js")
         return new Response(getNavRuntime(), { headers: JS_HEADERS });
       if (pathname === "/__wrnexus/realtime.js")
diff --git a/scripts/lib/measure-runtime-size.ts b/scripts/lib/measure-runtime-size.ts
index c5879e97..4dae9a6d 100644
--- a/scripts/lib/measure-runtime-size.ts
+++ b/scripts/lib/measure-runtime-size.ts
@@ -12,12 +12,14 @@
  */
 import {
   getReactiveRuntime,
+  getComponentControllerRuntime,
   getNavRuntime,
   getRealtimeRuntime,
 } from "../../packages/csr/src/index.ts";
 
 const runtimes: Record<string, string> = {
   "reactive-runtime.ts": getReactiveRuntime(),
+  "component-controllers.ts": getComponentControllerRuntime(),
   "nav-runtime.ts": getNavRuntime(),
   "realtime-runtime.ts": getRealtimeRuntime(),
 };
diff --git a/scripts/security-performance-audit.mjs b/scripts/security-performance-audit.mjs
index bd30e38f..1a8cc729 100644
--- a/scripts/security-performance-audit.mjs
+++ b/scripts/security-performance-audit.mjs
@@ -169,7 +169,8 @@ addCheck(
  * minified transfer once. That is the number worth defending.
  */
 const runtimeBudgets = {
-  "reactive-runtime.ts": 71_000,
+  "reactive-runtime.ts": 49_000,
+  "component-controllers.ts": 24_100,
   "nav-runtime.ts": 12_000,
   "realtime-runtime.ts": 8_000,
 };
@@ -182,7 +183,8 @@ const minifiedSizes = JSON.parse(
 );
 for (const [file, budget] of Object.entries(runtimeBudgets)) {
   const bytes = minifiedSizes[file];
-  const raw = statSync(join(root, "packages", "csr", "src", file)).size;
+  const sourceFile = file === "component-controllers.ts" ? "reactive-runtime.ts" : file;
+  const raw = statSync(join(root, "packages", "csr", "src", sourceFile)).size;
   addCheck(
     `PERF-RUNTIME-SIZE-${file.replace(/-runtime\.ts$/, "").toUpperCase()}`,
     typeof bytes === "number" && bytes <= budget,