Tony-
I suspect this entry in the What's New in Igor Pro 9 help file is relevant:
Prior to Igor Pro 9, a ListBox control did not respond to mouse clicks if the control was empty. Now an action procedure for an empty ListBox control will receive a mouse down event (event code 1, see WMListboxAction). Along with this change, we fixed a bug: an empty ListBox control with header cells and the userColumnResize keyword set to non-zero will now allow you to drag the cell divider to resize the columns.
Both of these require allowing the mouse-down to propagate further into the code. No doubt it is also causing focus to move to the listbox. Unless it's a real problem, I'm inclined to say this is intended, permanent, and cross-platform.