diff --git a/README.md b/README.md
index f845f99..1622e89 100644
--- a/README.md
+++ b/README.md
@@ -113,7 +113,9 @@ import BigNumber from './path/to/bignumber.mjs';
 
 ![External import](docs/images/external_import.png)
 
-Currently every external is loaded once and cached until Leporello is restarted (TODO what happens if we load modules in iframe and then recreate iframe)
+Currently every external is loaded once and cached until Leporello is restarted 
+(TODO what happens if we load modules in iframe and then recreate iframe)
+(TODO serve modules from service worker, change host every time)
 
 ## Hotkeys
 
@@ -123,6 +125,10 @@ See built-in Help
 
 Editing local files is possible via [File System Access API](https://developer.mozilla.org/en-US/docs/Web/API/File_System_Access_API). Click "Allow access to local project folder" to grant access to local directory.
 
+## Run and debug UI code in separate window
+
+TODO
+
 
 ## Run Leporello locally
 To run it locally, you need to clone repo to local folder and serve it via HTTPS protocol (HTTPS is required by [File System Access API](https://developer.mozilla.org/en-US/docs/Web/API/File_System_Access_API)). See [How to use HTTPS for local development](https://web.dev/how-to-use-local-https/)
diff --git a/docs/examples/preact/index.js b/docs/examples/preact/index.js
new file mode 100644
index 0000000..5a9ec7d
--- /dev/null
+++ b/docs/examples/preact/index.js
@@ -0,0 +1,23 @@
+/* external */
+import {h, render} from 'https://unpkg.com/preact?module';
+
+/* external */
+import {Stateful} from './stateful.js'
+
+const Counter = Stateful({
+  getInitialState: () => ({counter: 0}),
+
+  handlers: {
+    inc: ({counter}) => ({counter: counter + 1}),
+    dec: ({counter}) => ({counter: counter - 1}),
+  },
+  
+  render: (props, state, handlers) => 
+    h('div', null,
+      h('span', null, state.counter),
+      h('button', {onClick: handlers.inc}, 'Increment'),
+      h('button', {onClick: handlers.dec}, 'Decrement'),
+    )
+})
+
+render(h(Counter), globalThis.document.body)
diff --git a/docs/examples/preact/stateful.js b/docs/examples/preact/stateful.js
new file mode 100644
index 0000000..3d835a9
--- /dev/null
+++ b/docs/examples/preact/stateful.js
@@ -0,0 +1,31 @@
+import {Component} from 'https://unpkg.com/preact?module';
+
+export const Stateful = ({getInitialState, handlers, render}) => {
+
+  return class extends Component {
+
+    constructor() {
+      super()
+      this.compState = getInitialState()
+      this.handlers = Object.fromEntries(
+        Object
+          .entries(handlers)
+          .map(([name, h]) => 
+            [name, this.makeHandler(h)]
+          )
+      )
+    }
+
+    makeHandler(h) {
+      return (...args) => {
+        this.compState = h(this.compState, ...args)
+        this.forceUpdate()
+      }
+    }
+
+    render() {
+      return render(this.props, this.compState, this.handlers)
+    }
+  }
+
+}
diff --git a/src/calltree.js b/src/calltree.js
index 8882b3f..237634e 100644
--- a/src/calltree.js
+++ b/src/calltree.js
@@ -447,10 +447,15 @@ nodes that are in the second tree that are not in the first tree
 */
 const merge_calltrees = (prev, next) => {
   return Object.fromEntries(
-    Object.entries(prev).map(([module, {exports, calls}]) =>
+    Object.entries(prev).map(([module, {is_external, exports, calls}]) =>
       [
         module, 
-        {exports, calls: merge_calltree_nodes(calls, next[module].calls)[1]}
+        is_external
+          ? {is_external, exports}
+          : {
+              exports, 
+              calls: merge_calltree_nodes(calls, next[module].calls)[1]
+            }
       ]
     )
   )
diff --git a/src/cmd.js b/src/cmd.js
index 0949701..b643c9e 100644
--- a/src/cmd.js
+++ b/src/cmd.js
@@ -71,6 +71,7 @@ const run_code = (s, index, dirty_files) => {
     ...s,
     parse_result,
     calltree: null,
+    async_calls: null,
 
     // Shows that calltree is brand new and requires entire rerender
     calltree_changed_token: {},
@@ -82,7 +83,6 @@ const run_code = (s, index, dirty_files) => {
     calltree_node_is_expanded: null,
     frames: null,
     calltree_node_by_loc: null,
-    // TODO keep selection_state?
     selection_state: null,
     loading_external_imports_state: null,
   }
@@ -734,8 +734,10 @@ const move_cursor = (s, index) => {
   return do_move_cursor(state, index)
 }
 
-const on_async_call = (state, ...args) => {
-  console.log('on_async_call', state, args)
+const on_async_call = (state, call) => {
+  return {...state, 
+    async_calls: [...(state.async_calls ?? []), call]
+  }
 }
 
 const load_dir = (state, dir) => {
diff --git a/src/eval.js b/src/eval.js
index 5fb4b4b..044ebde 100644
--- a/src/eval.js
+++ b/src/eval.js
@@ -391,7 +391,12 @@ export const eval_modules = (
           is_toplevel_call = is_toplevel_call_copy
 
           if(is_recording_async_calls && is_toplevel_call) {
-            on_async_call(children)
+            if(children.length != 1) {
+              throw new Error('illegal state')
+            }
+            const call = children[0]
+            children = null
+            on_async_call(call)
           }
         }
       }
@@ -542,13 +547,7 @@ export const eval_modules = (
     ,
 
     /* on_async_call */
-    calls => {
-      on_async_call(
-        calls.map(c => 
-          assign_code(modules, c)
-        )
-      )
-    },
+    call => on_async_call(assign_code(modules, call))
   )
 
   const calltree_actions =  {
diff --git a/src/index.js b/src/index.js
index 818d45c..d5fdd56 100644
--- a/src/index.js
+++ b/src/index.js
@@ -10,6 +10,17 @@ const EXAMPLE = `const fib = n =>
     : fib(n - 1) + fib(n - 2)
 fib(6)`
 
+
+// By default run code in hidden iframe, until user explicitly opens visible
+// window
+globalThis.run_window = (() => {
+  const iframe = document.createElement('iframe')
+  iframe.src = 'about:blank'
+  iframe.setAttribute('hidden', '')
+  document.body.appendChild(iframe)
+  return iframe.contentWindow
+})()
+
 export const open_run_window = () => {
   if(globalThis.run_window != null) {
     globalThis.run_window.close()
diff --git a/test/test.js b/test/test.js
index 0ee167d..fb285a1 100644
--- a/test/test.js
+++ b/test/test.js
@@ -2306,11 +2306,27 @@ const y = x()`
       // Use Function constructor to exec impure code for testing
       new Function('fn', 'globalThis.__run_async_call = fn')(fn)
     `
-    const i = test_initial_state(code, {
-      on_async_call: (calls) => {console.log('test on async call', calls)}
-    })
-    globalThis.__run_async_call()
-    //delete globalThis.__run_async_call
+
+    const {get_async_call, on_async_call} = (new Function(`
+      let call
+      return {
+        get_async_call() {
+          return call
+        },
+        on_async_call(_call) {
+          call = _call
+        }
+      }
+    `))()
+
+    const i = test_initial_state(code, { on_async_call })
+    globalThis.__run_async_call(10)
+    const call = get_async_call()
+    assert_equal(call.fn.name, 'fn')
+    assert_equal(call.code.index, code.indexOf('() => {'))
+    assert_equal(call.args, [10])
+    const state = COMMANDS.on_async_call(i, call)
+    assert_equal(state.async_calls, [call])
   }),
 
 ]