See attached sample. Under 4.8.3, this program works correctly, that is, if you click in the window "Child", this window will be "reparented" to "Parent 1".
Or, as a second test case, if you change "new TChild(window2, window)"
for "new TChild(window2, 0)" then the Child window will become free of any parent window.

But in Qt 5.0.2, the same sample has no effect: the Child window remains locked to "Parent 2" in all cases. It becomes impossible to reparent any floating window
I could not find a fix inside Qt code, but as an hint, if you uncomment the line SetWindowLongPtr... then the program works correctly...