The ComboBox selection popup has the same size as the ComboBox control. When one of the selection entries ("DDDD" in the example") needs more space than the combobox size then the item is truncated. I think it would be more convenient to scale the selection dialog within the available space on the right side.

RowLayout {
	width: parent.width

	Label {
		text: qsTr("Test") + ":"
	}
	ComboBox {
		model:["A", "BB", "CCC", "DDDD"]
		Layout.fillWidth: true
	}

	Button {
		text: "P"
		Layout.fillWidth: true
	}
}