Hide menustrip in child form.
Otherwise there will be a gap left by this empty menuStrip.
Merge index can’t jump
MSDN How to: Insert a MenuStrip into an MDI Drop-Down Menu (Windows Forms) (MatchOnly for menu (root), and Insert for menu items.)
See the example, no gap between each menu items merge index. No Gap! There is different than PB and VB.
No menustrip inheritance?
MS forces you to use merge menus between mdi parent and child.