We know that force an application landscape on Windows Phone 8.1 we should follow the step like that:
http://qt-project.org/forums/viewthread/50668/
And to set this in AppxManifest.xml, we need to add the following configuration in AppxManifest.xml:

<m3:InitialRotationPreference>
<m3:Rotation Preference="landscape" />
</m3:InitialRotationPreference>

Now comes the question: when I set this configuration in AppxManifest.xml, and run my application on Windows Phone 8.1, the orientation of system is indeed landscape whereas the content of Qt Quick( or QML Window ) is not landscape. worse still any MouseArea in original relative position are becoming invalid.