We seemingly don't have a way to do QWindows with an alpha channel (ie arbitrary opacity, not just per window uniform opacity) on XCB. See https://bugreports.qt.io/browse/QTBUG-116183?focusedId=739850&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-739850

We do have some trick in the system tray implementation to copy the content from the underlying window, but it's unclear if that can work for GL/Vulkan as well.