Been there, I honestly couldn’t find a good solution.
One might think On Losing Focus + “GOTO OBJECT”, but of course, then you won’t be able to get out of the subform from the first and last objects (clicking is also a way of getting/losing focus). It is also not possible to know which direction the tab is moving.
Next, you’d think “capture tab and shift+tab keyboard shortcuts, instead of processing On Getting/Losing Focus”, but that requires a pair of hidden buttons whose behaviour you would toggle when the first or last object in the tab order has focus.
You might get the desires result, but then again, you have an enclosed circular tab system in the subform, which you can tab into but not tab out of.
So actually, for the first and last object, you need to differentiate a tabbing in from the parent form versus a return tab from the other end in the subform. And that’s when you start to think about giving up…