After testing this on other browsers I discovered that it is a bug in the old Safari 5 on my Snow Leopard. And it is only in HTML5 not Flash.
On my new iMac Retina it does not happen. And I have tested it in both Chrome and Firefox on Mac and PC and there is no problem.
However I discovered another bug.
When you use textfield as a child of a background with an png file autoheight fails in flash.
The textfield adjusts to a height that is just 20% of the background parent file. vcenter setting does not affect it.
And it is same in all browsers.
Any explanation Klaus.