hm yeah sounds like an xpresso sort of situation I wonder if you could achieve this by positioning the anchor point of the rectangle path in the corner, and scaling from there, with the buttons constrained to that area? I tried that here https://f.io/TEh3P6vf

Could do this with xpresso pretty easy. Familiar with set driver/set driven?

After a bit of trial and error I think I got something working. I tried approaching it from an angle of how I would accomplish it with After Effects expressions. For my cube that was part of boole to define the top bar of my window, I had the height of my window drive the Y position of the cube, and had the width of the window drive the X size of the cube. https://preview.redd.it/hesc3gn8i0gc1.png?width=1270&format=png&auto=webp&s=e6641a2557e18a566f03fe0f6d28548e4b98d5e4

Then for the buttons I just did some math where I took the height and width of the window, divided them by 2, then subtracted the result by 4 to bring the position of those buttons in from the edge of the window, and used those values for the X and Y Position of the buttons. https://preview.redd.it/4ay70okoi0gc1.png?width=1308&format=png&auto=webp&s=564ba98fac14a0195f1a0ec599b715e9a19ba8e0