When opening the drop down menu of a ComboBox in Material style, the 'enter' transition animation is missing.
Also, the ComboBox.popup's opacity is reset to 1 after the 'exit' transition, even though it was never set explicitly. Such behavior doesn't bother me so far, but just seem to be a little bit counterintuitive.
Should be related to the changes in I2b192c96eaea2906d968341255e80cd19be177e6.