This question came in from someone who was using our Flex Mobile DropDownList. I was writing up the issue for our product manual, but decided to blog it too.
The problem was that, in Flex 4.6, when opening the DropDownList's drop down text fields would display on top of them. After some research I was able to discover the problem.
The default skin for a TextInput in Flex 4.6 uses the StyleableStageText. StageText is always drawn above the Flash Display List, which is why the drop down's popup appears to be under it. To quote the Adobe docs on StageText:
Native text input fields cannot be clipped by other Flex content and are rendered in a layer above the Stage. Because of this limitation, components that use StageText-based skin classes will always appear to be on top of other Flex components. Flex popups and drop-downs will also be obscured by any visible native text fields. Finally, native text fields' relative z-order cannot be controlled by the application.
There are two possible solutions for this. The first is to architect your mobile application so that popups never display above a TextInput. This may be tough if you're building for multiple devices with different screen sizes.
The second is to use the Flex 4.5 skin class for your TextInputs:
The Flex 4.5 Skin Class does not use StageText under the hood, so will not exhibit the problem of the TextField positioning itself above any of your popups.