Application.OnWindow Property (Excel)
Returns or sets the name of the procedure that’s run whenever you activate a window. Read/write String.
The procedure specified by this property isn’t run when other procedures switch to the window or when a command to switch to a window is received through a DDE channel. Instead, the procedure responds to the user's actions, such as clicking a window with the mouse.
If a worksheet or macro sheet has an Auto_Activate or Auto_Deactivate macro defined for it, those macros will be run after the procedure specified by the OnWindow property.