You don’t have to put anything in the global area other than your navigation bloc - even that isn’t mandatory if you want to change your menu on each page. As you correctly point out, anything that sits in the top global area will be carried across all pages. However, the original issue is that by only adding one navigation bloc to the page and nothing else, you are effectively creating a page that is the height of the navigation bloc only. As you add content to the page (in the dynamic area), your page becomes longer. You need the extra page height to display a drop down navigation. If you don’t do this, the drop down will not show beyond the actual page bottom. This is best illustrated as shown below. I’ve added a navigation bar into the dynamic area of the page so you can see how page height is determined. IF I leave the top global area blank and the global footer blank, my page height will be determined by the height of the content I’ve placed in the dynamic area of the page. I’ve given the block a black background so you can see what happens in a browser window.
As you can see, in the blocs interface, it looks as if there is space above and below our navigation bloc. However, when previewing in a browser, things look a bit different.
If I now add a content bloc to the dynamic area of the page and put something in the top global area, everything will show up as expected and the page will be increased in size.
The same will happen if you place your navigation in the top global area. As soon as you add something to the dynamic area, the page will increase in length, thus allowing room for the drop-down menu. Effectively, the drop-down is floating over the page, but if there is no page below the navigation bloc, the dropdown will be prevented from fully showing because it will hit the page bottom.
So, if you want your menu in the top global area, go for that option. All you have to remember is to put some content in the dynamic area also to make your page long enough for the drop-down to display. It usually only needs one bloc to make sufficient room for the drop-down. But, of course, a real web page is likely to have several blocs anyway.