visibleFraction

pub fn visibleFraction(self: ScrollInfo, dir: enums.Direction) f32 {
    const viewport_start = switch (dir) {
        .vertical => self.viewport.y,
        .horizontal => self.viewport.x,
    };
    const viewport_size = switch (dir) {
        .vertical => self.viewport.h,
        .horizontal => self.viewport.w,
    };
    const virtual_size = switch (dir) {
        .vertical => self.virtual_size.h,
        .horizontal => self.virtual_size.w,
    };

    if (viewport_size == 0) return 1.0;

    const max_hard_scroll = self.scrollMax(dir);
    var length = @max(viewport_size, virtual_size);
    if (viewport_start < 0) {
        // temporarily adding the dead space we are showing
        length += -viewport_start;
    } else if (viewport_start > max_hard_scroll) {
        length += (viewport_start - max_hard_scroll);
    }

    return viewport_size / length; // <= 1
}