Which event is fired when I navigate to any other application in my PC?

I have a Custom Control of following type in .NET C# Winforms

 public partial class AutoSuggestPopupList : ToolStripDropDown
 {
 }

It has got all its properties and methods. I am using it with a ListBoxControl and a textbox/combobox.

It is used in following manner: 1. As user types something in textbox/combobox 2. the list of ListBoxControl is refreshed and accordingly the items of AutoSuggestPopupList are refreshed.

I am closing my AutoSuggestPopupList in different Event Handlers and it gets closed appropriately. But my only problem is when I navigate to any other software like Word or Excel from my App, the AutoSuggestPopupList is not closed and is fixed at its location.

I have already tried using frm_LocationChanged and frm_VisibleChanged Event Handlers, but of no use :(

It will be a great help if some one can tell me which event handler will be useful for me , when i navigate to any other software.