After merging #155539

Since OverlayPortal.targetsRootOverlay is now used, the root Overlay must be used when calculating the position for _Submenu

final RenderBox overlay = Overlay.of(
  anchorContext, 
  rootOverlay: true // NEED ADD THIS
).context.findRenderObject()! as RenderBox;

Line to need fixed

How to fix
add rootOverlay: true

How to reproduce the problem
Create a nested navigator with a height less than the screen height and put DropdownMenu there