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You need to write a resize handler for the status bar (similar to the one you have for the Treeview).

Make the Status bar a child and a member variable of the MainApp (just like the Treeview).
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I can't see the code where you move the status window (only the Treeview).

Is the status bar a child of the dialog or the treeview? (appears to be treeview where it should be the Dialig, or...

Can you post your OnSize() handler?