[Qt5 regression] Dropping items on non-Qt programs via DND does not work.

The problem:
Dragging an item from a Qt 5 program and dropping it on another non-Qt program does not work.
How to reproduce:
Launch qtbase/examples/widgets/draganddrop/draggabletext, and try to drag an text item in it, and then drop it on a non-Qt text editor (written in gtk2, gtk3, or FLTK).
Expected behavior: the dragged text should be inserted to the target editor.
Current behavior: nothing happens, the DND is cancelled and I always got this console error message: "QXcbDrag::handleFinished - drop data has expired"

p.s.: I'm a developer of LXQt desktop environment. We noted this problem because after upgrading to Qt 5.4, the DND in our file manager pcmanfm-qt stop working. Further tests showed that all cross-process DND operations are broken. Dropping on another Qt 5 program works sometimes, though.
This is a critical issue that breaks many desktop programs.

Thank you for the hard work.