Routing within subpath of another route may inherit content from parent route.

Hola,

I've been getting used to doing D8 development by porting a module over to D8 and in doing so, I found a strange bug (? I can only assume its a bug) within the routing system.

So in my module, I have a path (/admin/appearance/manage/google_webfont_loader_api) which is an administrative page within the appearance section.

However, when I first got rid of hook_menu and solely used the routing.yml file, I found out that the 'Google Webfont Loader Settings' tab was now gone (I was able to bring it back by implementing hook_menu again - why both routing and hook_menu are necessary when we used to have solely hook_menu perplexes me but as far as I have been told to accept, "it is what it is"). But last week, I found out that even though the menu item is missing, the route is still accessible. So I went to take a look and I found an interesting result:

(These are just unedited screenshots but I hope it is easy enough to see the different)
The parent item (at admin/appearance) seems to inject some of its own content onto the page (along with setting the menu item tab to 'list'). So while the routing/menu stuff may be intentional, this part of the behavior certainly seems like a bug. I'm happy to help provide any other information as necessary.