I am using Angular Material sidenav and my menu has a sub menu. When I open the sub menu a scrollbar appears making the sidenav slightly larger and it covers some of the main content.
See this Stackblitz https://stackblitz.com/edit/angular-raedvz, expand the sub menu and you will see the behavior.
Adding the autosize
attribute should fix your problem. Though that seems to have a performance impact.
Have a look at this github issue: https://github.com/angular/material2/issues/9601