Reader DC v2020.012.20048
Windows 10 10.0.19042 Build 19042, IE 11.572.19041

PDFs embeddd in IE 11, or using the "Preview PDF" tab in Livecycle designer, no longer show user dialogs triggered using "xha.host.messageBox" or "app.alert". Other "xfa.host" functions still return data. When opening the PDF 'stand alone' everything works as expected.

Here's an simple example form. The button should show two message dialogs as well as information in a text box. Interestingly the code continues to execute after the calls to xfa.host.messageBox and app.alert, and the text box is correctly populated.

Test page: https://www.aduk.net/pdfTest/

I have used the Acrobat DC removal tool and re-installed Acrobat.

These builds of Windows and IE 11 work:
10.0.17763 Build 17763, IE 11.1518.17763
10.0.18362 Build 18362, IE 11.1139.18362

These ones fail:
10.0.19041 Build 19041, IE 11.572.19041
10.0.19042 Build 19042, IE 11.572.19041