A 3rd party app can add an element in the atl.general webitem panel which lies above the page header. See the attached image where there's a toolbar above the page header.
 
When the user scrolls down, there will be a gap above the page header since the page header is fixed with the new side nav changes. See attached image. 
 
The page header should instead scroll up with the page like it used to (attached a gif). Or Ideally, the page header stays fixed on the screen but scrolls up to cover the gap.