Qt 5.15.4 Known Issues
From Qt Wiki
All Qt 5.15.4 LTS Commercial known issues are listed the release note that is available via Qt Account. In Qt Account, select Downloads - Product: Qt - Version: 5.15.4.
- Check that your system meets Qt's requirements
- All open issues can be found from Jira: https://bugreports.qt.io
- RTA findings here
The PDF viewer switched in 5.15.4 to a different embedding model that more closely resembles how Chrome works. This means navigating to a PDF will no longer change the URL to an extension link, and PDF document itself will be shown in an embed element on the page. One side effect of this is that you can no longer scroll the PDF viewer with window.scrollTo, see https://bugreports.qt.io/browse/QTBUG-93871