This is easiest to test if you have installed the sample data. These test instructions depend on the presence of the sample data.

1. Create a new menu item in the "main" menu called "test". Save and close.

2. Re-open the "test" menu item and move it to the "main menu blog" by selecting the new menu on the right hand side - do not save just yet

3. The parent item dropdown is now populated with the contents of the "main menu blog".

4. Select "Sample Layouts" as the parent.

5. Save and close

Expected behaviour

The "test" menu item is in the "main menu blog" and it is a submenu of "sample layouts"

Actual behaviour

The "test" menu item is in the "main menu blog" and it is a toplevel menu item