We started to test the 5.4 Beta today and in it, we have problems with the ComboBox.
It is there, and shows the correct initial value, but clicking on it doesn't open it. There is some slight shifting of pixels, so something happens, but it is otherwise completely unusable.
All our test machines run some form of Kubuntu 14.04, and all our production machines have Intel HD4000 graphics. Some of the development PCs have Nvidia cards, and there the ComboBoxes seems to work as expected.
It reminds me of
QTBUG-41666, but we don't have problems with the ComboBox in our appliction in 5.3.2.
I was thinking that this may have to do with the announced update of shaders to support the 3.3 core profile and above since my experience with shaders is that the Mesa Intel drivers are more strict and/or less robust than when using the same shaders on Nvidia hardware.