SmoothMouseDragging.cs

/**
This script is created by Şahin Kasap (github.com/sahin52)
This script contains an example for smooth and clean dragging when colliders exist on a scene.
Normally, when we just add dragging functionality, even though the colliders exist, the objects passes through colliders 
(there becomes a small bumb but it is not enough), so I added the colliders tags, and added the tags to a List.
If the collider is in the list and the mouse position is on the other side of the collider, I make the velocity to the other position 0. 
Not a perfect solution but works fine in my case.
*/

using System.Collections.Generic;

using UnityEngine;

class SmoothMouseDragging: MonoBehaviour{


    List<string> collideds = new List<string>();
    private Rigidbody rb;
    void Awake(){
        rb = GetComponent<Rigidbody>();
    }
    void FixedUpdate(){
        var mousePos =  Camera.main.ScreenToWorldPoint(Input.mousePosition);
        mousePos.z = transform.position.z;
        var velocity = mousePos-transform.position;
        if(collideds.Contains("borderTop") && velocity.y>0f){
            print("cancelling upwards speed");
            velocity.y = 0;
        }
        if(collideds.Contains("borderLeft") && velocity.x<0f){
            print("cancelling left speed");
            velocity.x = 0;
        }
        if(collideds.Contains("borderBottom") && velocity.y<0){
            print("cancelling downwards spped");
            velocity.y=0;
        }
        rb.velocity = velocity*speed;
    }

    void OnCollisionEnter(Collision collision){
        if(collideds.IndexOf(collision.collider.tag)<0)
            collideds.Add(collision.collider.tag);
    }
    void OnCollisionExit(Collision other){
        if(collideds.IndexOf(other.collider.tag)>-1){
            collideds.RemoveAt(collideds.IndexOf(other.collider.tag));
        }
    }
}