Since we can't use traditional event handlers in the print iframe when

JS is blocked, we consider it safe to discard of the hidden iframe once
event handling has returned to the main content window.

packages/apputils/src/printing.ts

@@ -93,9 +93,10 @@ export namespace Printing {
       iframe.src = 'about:blank';
       setIFrameNode(iframe, textOrEl as HTMLElement);
     }
-    const printed = resolveAfterPrint(iframe);
-
+    const printed = resolveAfterEvent();
     launchPrint(iframe.contentWindow);
+    // Once the print dialog has been dismissed, we regain event handling,
+    // and it should be safe to discard the hidden iframe.
     await printed;
     parent.removeChild(iframe);
   }
@@ -140,9 +141,25 @@ export namespace Printing {
     });
   }
 
-  function resolveAfterPrint(iframe: HTMLIFrameElement): Promise<void> {
+  /**
+   * A promise that resolves after the next mousedown, mousemove, or
+   * keydown event. We use this as a proxy for determining when the
+   * main window has regained control after the print dialog is removed.
+   *
+   * We can't use the usual window.onafterprint handler because we
+   * disallow Javascript execution in the print iframe.
+   */
+  function resolveAfterEvent(): Promise<void> {
     return new Promise(resolve => {
-      iframe.contentWindow.onafterprint = () => resolve();
+      const onEvent = () => {
+        document.removeEventListener('mousemove', onEvent, true);
+        document.removeEventListener('mousedown', onEvent, true);
+        document.removeEventListener('keydown', onEvent, true);
+        resolve();
+      };
+      document.addEventListener('mousemove', onEvent, true);
+      document.addEventListener('mousedown', onEvent, true);
+      document.addEventListener('keydown', onEvent, true);
     });
   }
 
@@ -151,8 +168,7 @@ export namespace Printing {
    */
   function launchPrint(contentWindow: Window) {
     const result = contentWindow.document.execCommand('print', false, null);
-
-    // exeCommand won't work in firefox so we call the `print` method instead if it fails
+    // execCommand won't work in firefox so we call the `print` method instead if it fails
     // https://github.com/joseluisq/printd/blob/eb7948d602583c055ab6dee3ee294b6a421da4b6/src/index.ts#L148
     if (!result) {
       contentWindow.print();