Custom shader does not update if masked,

I have an issue that has been partially addressed elsewhere (like http://answers.unity3d.com/questions/980924/ui-mask-with-shader.html and http://answers.unity3d.com/questions/1004826/custom-ui-material-with-mask.html), but in my case i am trying to also update the shader, and this solution : http://answers.unity3d.com/questions/1130203/ui-mask-override-my-shaders-custom-property.html did nothing in my case.

This is my shader :

 Shader "MyLinearGradient"
 {
     Properties{
         _TopColor("Top color", Color) = (1,1,1,1)
         _BottomColor("Bottom color", Color) = (1,1,1,1)
         _Offset("Offset",Float) = 0
 
         // required for UI.Mask
         _StencilComp("Stencil Comparison", Float) = 8
         _Stencil("Stencil ID", Float) = 0
         _StencilOp("Stencil Operation", Float) = 0
         _StencilWriteMask("Stencil Write Mask", Float) = 255
         _StencilReadMask("Stencil Read Mask", Float) = 255
         _ColorMask("Color Mask", Float) = 15
     }
         SubShader
     {
         Tags
         {
             "Queue" = "Transparent"
             "IgnoreProjector" = "True"
             "RenderType" = "Transparent"
             "PreviewType" = "Plane"
             "CanUseSpriteAtlas" = "True"
         }
         LOD 100
 
         Stencil
         {
             Ref[_Stencil]
             Comp[_StencilComp]
             Pass[_StencilOp]
             ReadMask[_StencilReadMask]
             WriteMask[_StencilWriteMask]
         }
         ColorMask[_ColorMask]
         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
 
             #include "UnityCG.cginc"
 
             fixed4 _TopColor;
             fixed4 _BottomColor;
             float1 _Offset;
 
             struct v2f {
                 float4 position : SV_POSITION;
                 fixed4 color : COLOR;
             };
 
             v2f vert(appdata_full v)
             {
                 v2f o;
                 o.position = mul(UNITY_MATRIX_MVP, v.vertex);
                 float1 IntOffset = v.texcoord.y + v.texcoord.x + _Offset;
                 o.color.r = cos(IntOffset)*(_TopColor.r - _BottomColor.r) + _BottomColor.r;
                 o.color.g = cos(IntOffset)*(_TopColor.g - _BottomColor.g) + _BottomColor.g;
                 o.color.b = cos(IntOffset)*(_TopColor.b - _BottomColor.b) + _BottomColor.b;
                 o.color.a = cos(IntOffset)*(_TopColor.a - _BottomColor.a) + _BottomColor.a;
                 return o;
             }
 
             fixed4 frag(v2f i) : SV_Target
             {
                 float4 color = i.color;
                 color.a = 1;
                 return color;
             }
             ENDCG
         }
     }
 }
 

which i try to slide with the _offset parameter. The material works fine except if it is under a ui mask. i also attach a test project in order to see it live.

Thank you in advance

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