Currently, the invariant is false for all `percentFilled != 0.5 && angle != 0`. I believe this is because of the x-values of the points

FluidView.swift

/// A view representing a dynamically moving fluid with a quadrilateral
/// - Invariant: The area of the view is exactly equal to the area of the rectangle of the parent view times `percentFilled`
struct FluidView: Shape {
    var angle: CGFloat = 0.0
    var percentFilled: CGFloat = 0

    var animatableData: CGFloat {
        get { angle }
        set { self.angle = newValue }
    }
    
    /// Creates a new FluidView
    /// - Parameters:
    ///   - angle: A value in the range `0...1`. A value of `0` indicates the view is horizontal, and an angle of `1` indicates the view is vertical (horizontal if viewed as landscape)
    ///   - percentFilled: the amount of the view bounds to fill represented as a value in the range `0...1`. A value of `x` indicates that `x * 100`% of the parent view is covered by this view
    init(angle: CGFloat = 0, percentFilled: CGFloat = 0) {
        precondition(0...1 ~= angle)
        precondition(0...1 ~= percentFilled)

        self.angle = angle
        self.percentFilled = percentFilled
    }

    func path(in rect: CGRect) -> Path {
        var path = Path()
        path.move(to: CGPoint(x: 0, y: rect.height * (1 - percentFilled))) // top left

        let lines = [
            CGPoint(x: 0, y: rect.height), // bottom left
            CGPoint(x: rect.width, y: rect.height), // bottom right
            CGPoint(x: rect.width, y: (rect.height * (1 - percentFilled)) * (1 + angle)), // top right
            CGPoint(x: 0, y: (rect.height * (1 - percentFilled)) * (1 - angle)) // top left
        ].map { pt in
            // make sure no points exceed the bounds
            CGPoint(x: pt.x, y: min(rect.height, pt.y))
        }

        // postcondition
        check(lines.area == rect.area * percentFilled) {
            "actual: \(lines.area), expected: \(rect.area * percentFilled)"
        }

        path.addLines(lines)
        return path
   }
}