Thanks for replying!
I know it's Reaper and not Logic Pro, but hey! The more similar, the better... don't you think?
A fluid layout is a layout that changes depending on the width and height of the document. Websites nowadays are responsive: they have the desktop, tablet and mobile layouts.
What I was trying to say is that when I enlarge bit by bit the mixer, there are moments when you find empty spaces that should be filled for instance with fx slots or fx returns.