I went from Qt 4.7.4 to Qt5.1.0.
There is now a strange thing happening with modal QDialogs that have the Qt::Tool flag, and all platforms are affected (Windows, Mac and Linux):
It seems as the modal dialog is not completely modal: one can select edit boxes, and even close the parent dialog by clicking its close button in the upper left corner. The consequence of this are regular crashes.
On top of that, customization flags don't seem to work well anymore, at least on Mac e.g:
Qt::Tool|Qt::CustomizeWindowHint|Qt::WindowTitleHint will still offer colored maximize and close buttons in the dialog bar.
Refer to the example project attached.