Collapsing TitleWindow fails on Linux, but not on Windows

Steps to reproduce:
1.create a custom component based on TitleWindow
2.set height="

{getHeaderHeight()}"
3.use this TitleWindow in an application
4.run the application

Actual Results:
On Linux : title and closeButton aren't visible
On WIndows : title and closeButton are visible (as a collapsed window)

Expected Results:
As on Windows, TitleWindow should show its titlebar

Workaround (if any):
Debugging on Linux is not possible now, since last AIR update, because Flex Builder Alpha doesn't work anymore (so it's impossible to know what's going on).

Code example :

THE TITLEWINDOW SUBCLASS :
<?xml version="1.0" encoding="utf-8"?>

<mx:TitleWindow xmlns:mx="http://www.adobe.com/2006/mxml" height="{getHeaderHeight()}

">

</mx:TitleWindow>

THE WINDOWED APPLICATION :
<?xml version="1.0" encoding="utf-8"?>

<mx:WindowedApplication xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" xmlns:ns1="com.*">

<ns1:ExtendedWindow x="50" y="50" width="150" title="Test" showCloseButton="true">

</ns1:ExtendedWindow>

</mx:WindowedApplication>