tab focus moving does not work as expected when showing alert in popup

[Problem]
=========
Tab focus moving does not work as expected when showing alert in a popup.

STEPS TO REPRODUCE:
===================
<Test1>
1. Run the sample project of Flex 4.6, run focusTet.mxml
2. click "show POPUP" button and in the popup, click "show alert" button, then click "cancel" in the alert.
3. press tab key.

<Test2>
1. Run the sample project of Flex 4.6, run focusTet.mxml
2. click "show POPUP" button and in the popup, click "show alert" button, then click "cancel" in the alert.
3. click "show alert" button again
4. press tab key.

RESULT:
=======
<Test1>
The tab focus moves to the text input in the main page, not just within the popup window.

<Test2>
The tab focus moves in the popup window

EXPECTED:
=========
<Test1>
The tab focus should only move within the popup window.

<Test2>
The tab focus should move within the alert window.

WORKAROUND:
=========
Not available