Your question is confusing. If you want an element to be 100% of the screen height, then .h-screen will do exactly that. A scroll bar appears because you have other elements present, so your document height is greater than the screen height.

If however, you just want the pink element to fill the remaining available space (which is always less than 100% of the screen height), then simply set that element to grow, using the flex property .grow and making that element a child of a .flex element.

…sometimes I cannot change the parent divs classes

Why not?