OK, figured it out. Followed MS's instructions on creating an event-handler object:



Using events with the Application object (Word) | Microsoft Docs

In case anyone else ever needs something like this, it was simple:

Create a class module:

Insert -> Class Module

In the View -> Properties window, give your new module a name. (I just used MS's example: EventClassModule )

In the class module, put this at the top to declare an object with events:

Click your new object ("App") in the Object dropdown, and you'll see all the interesting events you can use with your new object in the Procedures drop-down to its right -- there's lots! I picked the WindowActivate one for what I needed:

The last thing is to connect your new class object ("App") to the Word.Application object. Which is just this code in any module.
Run this Register_Event_Handler sub, and the Event becomes active. Each time I come back to the word doc from another app, it triggers! Perfect!

This was really useful, thank you.

I would like to know if there is a similar method for running a macro when a Document is activated or re-activated (rather than when the Word Application window itself is activated). Is there any way of doing that?

What I'm hoping for is something that works similarly to
Private Sub Worksheet_Activate()
for a sheet in an Excel workbook, so that the macro fires when a Word document is activated or re-activated while the Word Application remains running.

With thanks in anticipation...

If I'm understanding you correctly, it seems it would work if you use the "DocumentChange" procedure (in that second step) rather than WindowActivate. So the sub (in the class module) looks like this:



Private Sub App_DocumentChange()
MsgBox "changed!"
End Sub

Everything else remains the same. This fires the sub every time you switch from one doc to another without leaving Word.