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This should not happen when compositing is on, as the lower layer does not need to be drawed again. See 2D without compositing as one single image. If you alter (with a combobox) one part, then what was under it does not exist anymore, so it need to be recreated after the alteration (combobox) is gone.
With compositing, it is a stack. The image you see is composed of multiple layers, each being on top, or not of an other. They are separated and does not need to each other. If you remove a layer (your combobox), then it will just show a lower layer, no need to as the widget to replain itself (that is slow).
The widtget -layer- (plasma) is a layer and the QComboBox is a proxy Widget to a real QComboBox. The QComboBox create a popup window on top of the layer that created the window. So yes, a QComboBox poupup is composited.