Transparent Nav bar

[ghost]

If i set the Window to have a see through Nav bars it means that the FAB moves down (i would expect that) but how would i want to move it up so its above the nav bar AND when i scroll down it disappears not sitting lower no the screen..

Window window = getWindow();

window.setFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_NAVIGATION,WindowManager.LayoutParams.FLAG_TRANSLUCENT_NAVIGATION);