TextArea scrollbar tumb does not disappear , after the contents are selected (ctrl + A) & deleted.

Step to reproduce:
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1)Run the attached program.
2)Type some characters into the textarea(12 to 15 lines), so that you can see the vertical scrollbar tumb moving toward down arrow.
3)Select the contents of the textarea (Ctrl + A), you can see the contents of the textarea area selected.
4) Press 'Del' button on the keyboard. You can see the contents of the textarea are deleted , If you see the scrollbar tumb then the bug is reproduce.
5) Try Pressing on the upper arrow button of the vertical scrollbar of the textarea using mouse . you can see the tumb moves toward the upper arrow , once the mouse is released the tumb of the vertical scrollbar come back to the down, touching the down arrow of the scrollbar.

I tested the same screnario with the vista notepad, it work fine .
I have attached the screenshots which describes all the states. I tested this on Vista beta2 (build 5308) on Mustang b76.