this is a bit of a guess... I think it may be due to you using drawer.addItem
whereas I think what you want is drawer.triggerCustomView
. or possibly a combination of both.
Essentially drawer.addItem
is for declaring that you want an extension to be included in the drawer. When there are multiple extensions included in the drawer, when it opens it will then show you a menu of those items.
Typically you then run Adapt.drawer.triggerCustomView
to display your extension in the drawer in response to that button being clicked. But, when there's only one thing registered with the drawer, the behaviour is a bit different (because it'd be daft to show the drawer menu with just one item in it) so it goes straight into triggering the display for the sole drawer item... which is the bit you're missing, which is perhaps why it's getting a bit messed up.
Some relevant code snippets:
Glossary registering itself with the drawer.
Glossary triggering custom view when drawer glossary button clicked (or when drawer opened and glossary is the only thing in the drawer)
Hope this helps