MDI Validating

Hi,
I am facing a weird issue. I have a MDI application. The child form contains a textbox and there is a treeview control in the MDI Parent.
If I leave the textbox of the childform by clicking on the treeview, the validating event of the textbox is fired but the active control of the MDI Parent is the childform instead of being the treeview. Why ?
This sample illustrates my problem:

http://graphicsxp.free.fr/ValidatingSample.zip

Thank you for helping

[610 byte] By [graphicsxp] at [2007-12-17]