Subform popup (shortcut) menus

I have a main form with a subform in datasheet view.
When I right-click anywhere on the subform, I would like to display a custom
shortcut menu.
I have created my shortcut menu, and have set the properties of each
control, as well as the form, to display this custom form.
This works fine if I click anywhere in the Detail section of the subform, or
if I click on a RecordSelector.
However, if I click on the header at the top of any column, I get the full
default shortcut menu.

How can I get my custom shortcut menu to display when I click on a column
header?

TIA
- Turtle