Here is the full project, I have created a new project with minimal CSS and no CSS created by me. And the result is same.
Here is code of my Home class :
class Home extends React.Component{
state={};
render() {
return (
<div>
<div className="container">
<h1>Assignment Submission System</h1>
<p className="lead">To get latest notification, click
<tab>
<Link to={"/notifications"}>
here
</Link>
</tab>
.
</p>
<p>All enrolled class-rooms, assignment to-do list are shown.</p>
<h2 className="mt-4">Class-rooms:</h2>
<ClassRoomUnit displayName={"class 1"} classID={"4324ax"} hLink={"/class/asd"}/>
<ClassRoomUnit displayName={"class 2"} classID={"4324ax"} hLink={"/class/xzx"}/>
<ClassRoomUnit displayName={"class 3"} classID={"4324ax"} hLink={"/class/asd"}/>
<ClassRoomUnit displayName={"class 4"} classID={"4324ax"} hLink={"/class/asd"}/>
</div>
</div>
);
}
}
export default Home;
But the problem is, when there is only very few ClassRoomUnit item, it renders correctly :
But when I add a lot more ClassRoomUnit, first portion and many item disappears, although the scrollbar remains :

