Comment 0 for bug 1230091

(I'm filing this as a bug in order to be able to point other people to it, and to track its progress; if there's a blueprint containing this task, please let me know)

Some components (such as the Online Accounts trusted helper) need to be able to pop-up a window (typically, a dialog) on top of the running application. Such windows should be modal to the application, that is the user should not be able to interact with the application while the modal window is displayed on top of them. This also means that in the task switcher one shouldn't see two windows, but only the topmost modal window (and parts of the application window, in case the modal window on top is a non-fullscreen dialog).

For developers, this API already exists in Qt: see https://qt-project.org/doc/qt-5.1/qtgui/qwindow.html#fromWinId
It needs to be implemented in the QPA plugin, so feel free to add the relevant projects to the bug report.