diff --git a/src/btn.ts b/src/btn.ts
index 0bd6a9c..47b543f 100644
--- a/src/btn.ts
+++ b/src/btn.ts
@@ -1,7 +1,7 @@
 
 import { ScoreInfo } from './scoreinfo'
 import { loadMscore, WebMscore } from './mscore'
-import { useTimeout, windowOpen, console, attachShadow } from './utils'
+import { useTimeout, windowOpenAsync, console, attachShadow } from './utils'
 import i18n from './i18n'
 // @ts-ignore
 import btnListCss from './btn.css'
@@ -116,7 +116,7 @@ export class BtnList {
   /**
    * replace the template button with the list of new buttons
    */
-  commit (mode: BtnListMode = BtnListMode.InPage): void {
+  async commit (mode: BtnListMode = BtnListMode.InPage): Promise<void> {
     switch (mode) {
       case BtnListMode.InPage: {
         // fallback to BtnListMode.ExtWindow
@@ -148,7 +148,7 @@ export class BtnList {
 
       case BtnListMode.ExtWindow: {
         const div = this._commit()
-        const w = windowOpen('', undefined, 'resizable,width=230,height=270')
+        const w = await windowOpenAsync('', undefined, 'resizable,width=230,height=270')
         // eslint-disable-next-line no-unused-expressions
         w?.document.body.append(div)
         window.addEventListener('unload', () => w?.close())
@@ -176,7 +176,7 @@ export namespace BtnAction {
 
   export const openUrl = (url: UrlInput): BtnAction => {
     return process(async (): Promise<any> => {
-      windowOpen(await normalizeUrlInput(url))
+      return windowOpenAsync(await normalizeUrlInput(url))
     })
   }
 
@@ -195,7 +195,7 @@ export namespace BtnAction {
       btn.onclick = null
       setText(i18n('PROCESSING')())
 
-      const w = windowOpen('') as Window
+      const w = await windowOpenAsync('') as Window
       const txt = document.createTextNode(i18n('PROCESSING')())
       w.document.body.append(txt)
 
diff --git a/src/utils.ts b/src/utils.ts
index 857be1a..5fdaf79 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -47,6 +47,21 @@ export const useTimeout = async <T> (promise: T | Promise<T>, ms: number): Promi
   })
 }
 
+export const getSandboxWindowAsync = async (): Promise<Window> => {
+  if (typeof document === 'undefined') return {} as any as Window
+
+  return new Promise((resolve) => {
+    window.onmouseover = () => {
+      const iframe = document.createElement('iframe')
+      iframe.style.display = 'none'
+      document.body.append(iframe)
+      const w = iframe.contentWindow
+      window.onmouseover = null
+      resolve(w as Window)
+    }
+  })
+}
+
 export const getUnsafeWindow = (): Window => {
   // eslint-disable-next-line no-eval
   return window.eval('window') as Window
@@ -54,8 +69,8 @@ export const getUnsafeWindow = (): Window => {
 
 export const console: Console = (window || global).console // Object.is(window.console, unsafeWindow.console) == false
 
-export const windowOpen: Window['open'] = (...args): Window | null => {
-  return window.open(...args) // Object.is(window.open, unsafeWindow.open) == false
+export const windowOpenAsync = (...args: Parameters<Window['open']>): Promise<Window | null> => {
+  return getSandboxWindowAsync().then(w => w.open(...args))
 }
 
 export const attachShadow = (el: Element): ShadowRoot => {