unity surface shader that draws a transparent circle around the mouse (use case: to show what is behind a wall in an isometric game)

Hover.cs

    void Start()
    {
        theRenderer = GetComponent<MeshRenderer>();
        theRenderer.material.SetVector("_Resolution", new Vector4((float)Screen.width, (float)Screen.height, 0, 0));
    }

    void OnMouseEnter()
    {
        theRenderer.material = hoverMaterial;
        theRenderer.material.SetVector("_Resolution", new Vector4((float)Screen.width, (float)Screen.height, 0, 0));
    }

    void OnMouseExit()
    {
        theRenderer.material = normalMaterial;
    }

    void OnMouseOver()
    {
        //Debug.Log(Screen.width);
        theRenderer.material.SetVector("_MousePos", new Vector4((float)Input.mousePosition.x, (float)Input.mousePosition.y, 0, 0));
    }

transparentcircle.shader

Shader "Custom/transparentcircle"
{
    Properties
    {
        _Color ("Color", Color) = (1,1,1,1)
        _MainTex ("Albedo (RGB)", 2D) = "white" {}
        _Glossiness ("Smoothness", Range(0,1)) = 0.5
        _Metallic ("Metallic", Range(0,1)) = 0.0
        //_MousePos ("MousePos", Vector) = (0,0,0,0)
        _Resolution ("Resolution", Vector) = (800,600,0,0)
    }
    SubShader
    {
        Tags { "Queue" = "Transparent" "RenderType"="Opaque" }
        LOD 200

        CGPROGRAM
        // Physically based Standard lighting model, and enable shadows on all light types
        #pragma surface surf Standard fullforwardshadows alpha:fade //finalcolor:mycolor

        // Use shader model 3.0 target, to get nicer looking lighting
        #pragma target 3.0

        sampler2D _MainTex;

        struct Input
        {
            float2 uv_MainTex;
            float4 screenPos;
        };

        half _Glossiness;
        half _Metallic;
        fixed4 _Color;
        float2 _MousePos;
        float2 _Resolution;
        bool _enabled;

        // Add instancing support for this shader. You need to check 'Enable Instancing' on materials that use the shader.
        // See https://docs.unity3d.com/Manual/GPUInstancing.html for more information about instancing.
        // #pragma instancing_options assumeuniformscaling
        UNITY_INSTANCING_BUFFER_START(Props)
            // put more per-instance properties here
        UNITY_INSTANCING_BUFFER_END(Props)

        void surf (Input IN, inout SurfaceOutputStandard o)
        {
            // Albedo comes from a texture tinted by color
            fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
            o.Albedo = c.rgb;

            float2 screenUV = IN.screenPos.xy / IN.screenPos.w;
            screenUV.xy *= _Resolution;
            float2 MousePos = float2(_MousePos.x, _MousePos.y);
            float d = distance(MousePos, screenUV); // [0 .. 99999]
            float maxd = _Resolution.x / 14; // 100
            float feather = maxd / 6; // 10

            d = smoothstep(maxd-feather, maxd, d); // [0 0 0 0 0 90, / / / 100 1 1 1 1 1 1]

            // Metallic and smoothness come from slider variables
            o.Metallic = _Metallic;
            o.Smoothness = _Glossiness;


            o.Alpha = d;
        }
        ENDCG
    }
    FallBack "Diffuse"
}