ComboBox3.contentItem must be a QQuickTextInput to fix autocomplete
See QQuickComboBoxPrivate::updateEditText() for the logic:
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.
The wheel events still work, as they are captured by the MouseArea in
the background component.
When editable=true, the ComboBox previously closed the popup when
releasing the click when opening the dropdown. So you were required
to press+hold+drag.
This also fixes the controlRoot is undefined spam when press+hold+dragging.
controlRoot.activated(indexUnderMouse); was missing
MobileTextSelection.MobileTextActionsToolBar.controlRoot
Also see frameworks/qqc2-desktop-style!19 (merged) for the org.kde.desktop QQC2 patch.