I have found the sub menu items will be hidden behind the container while Menu was placed within PanelBar(overflow: auto).
SplitterPane have the same unexpected situation too.
Why the Menu do not just popup the sub menu items as ContextMenu ?
Menu / Context Menu
I hope the next (or next next) release will fix this elementary problem.
*z-index have no effects (because containers overflow: auto).
*containers autoflow: auto are required in my application (don't ask me use overflow: visible or larger width).