Fix inability to scroll context menu using scrollbar

When the root node of a focus event computation is a
dijitMenuPopup, use its contained widget as the root node instead.
Otherwise, the focus stack becomes empty and the context menu is
incorrectly closed.

Fixes #17435.

This works but I feel like this is a hacky approach; I wonder if it makes more sense to set the `dijitpopupparent` to the contained context menu instead.