NoIdeaWhatThatIs
Do you mean like a sticky nav?
Coz that'd be pretty cool.
Edit: Λssuming maybe something like this was occuring:
(1: Before scrolling down on mobile...)
(2: During scrolling down on mobile...)
Edit: if that's the same issue. Then what I did was clear out the standard Admin CP Custom Header HTML entirely and just use the Custom Header Extension instead.
That way the Nav Bar is consistently stuck to the top and doesn't do that ugly jumping thing.