ComboBox.contentItem must be a QQuickTextInput to fix autocomplete
See QQuickComboBoxPrivate::updateEditText()
https://github.com/qt/qtquickcontrols2/blob/dev/src/quicktemplates2/qquickcombobox.cpp#L426
By removing the MouseArea previously in the contentItem, we lose the
ability to press+hold+drag to select a drop down item.
A MouseArea has been added to the background component to capture
wheel events in a similar pattern to PlasmaComponents3.ComboBox.
The combobox previously closed the popup when releasing the click to
open the dropdown when editable=true.
Also see plasma-framework!36 (merged) for the PlasmaComponents3 patch.
BUG: 425865