Having a sidebar or not makes the NavigationBar act differently.
Something you can try:
Can you set MyTheme.NavigationBar("yourTheme").TopBarWide = True?
This should normally use approx. the whole width of the nav bar and the button should go way to the right.
Both questions have to do with the same concept: containers.
In ABM (actually Materialize CSS), a container will resize itself depending on the size of the browser. e.g. sometimes it will use 75% of the width, and if you make it smaller it will take 90% of the width of the browser. You can see this change when your resize the browser.
And for the body, if you set the centerInPage param in the AddRow methods to false, then the row will use 100% of the browsers width. If set to true then it will act as described above.