ToolTip Not shown when the Cbutton is just above the Windows TaskBar.

Steps to reproduce:
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1) Compile and run the attached testcase.
2) Once the windows is visible, take the frame near to task bar such that the button is just above TaskBar, Lower end of button almost touching the Taskbar.
3) Take Mouse over the Button.
4) Wait for some time ....
5) The tooltip does not appear. This is unlike Native
6) Now take the frame in the middle of the screen and take the mouse over the button. The ToolTip appears.

It is happening on WinXp also. I tested this example with B46 and B59.