Valeri,
I downloaded the example posted in this forum (
treeincombo-updated.zip), by Tihomir, and it has two combo boxes. The first showing the standard behavior, and the second showing the tree-view inside the combo box.
However, I just downloaded your sample and I'm getting similar behavior. Just perform the following steps:
1) Run your sample
2) Select the combo box drop down (don't select a parent item)
3) Click the tree expander to expand one parent
4) Select a child element
No item appears in the drop down.
In your example, once one parent is selected, then it seems you can select a child element on the next subsequent selection. However, if a child is already selected in the combo box, it prevents you from changing the selection to a different child.
Tihomir's example has a different issue in that selection a child shows all elements in the tree (see Yelong's screenshot).