update

Packages/app.rive.rive-unity/Runtime/Shaders/Resources/RiveUIDefault.shader

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+Shader "Rive/UI/Default"
+{
+    Properties
+    {
+        // Standard UI/Main texture (per renderer)
+        [PerRendererData] _MainTex ("Texture", 2D) = "white" {}
+        _Color ("Tint", Color) = (1,1,1,1)
+
+        // Stencil properties so Mask / RectMask2D work
+        [PerRendererData] _StencilComp ("Stencil Comparison", Float) = 8
+        [PerRendererData] _Stencil ("Stencil ID", Float) = 0
+        [PerRendererData] _StencilOp ("Stencil Operation", Float) = 0
+        [PerRendererData] _StencilWriteMask ("Stencil Write Mask", Float) = 255
+        [PerRendererData] _StencilReadMask ("Stencil Read Mask", Float) = 255
+
+        // Color mask / alpha clip for UI
+        [PerRendererData] _ColorMask ("Color Mask", Float) = 15
+        [PerRendererData] _UseUIAlphaClip ("Use Alpha Clip", Float) = 0
+    }
+
+    SubShader
+    {
+        Tags
+        {
+            "Queue"="Transparent"
+            "IgnoreProjector"="True"
+            "RenderType"="Transparent"
+            "PreviewType"="Plane"
+            "CanUseSpriteAtlas"="True"
+        }
+
+        // Stencil setup used by Mask / RectMask2D
+        Stencil
+        {
+            Ref [_Stencil]
+            Comp [_StencilComp]
+            Pass [_StencilOp]
+            ReadMask [_StencilReadMask]
+            WriteMask [_StencilWriteMask]
+        }
+
+        ColorMask [_ColorMask]
+
+        Cull Off
+        ZWrite Off
+        ZTest [unity_GUIZTestMode]
+
+
+        Blend One OneMinusSrcAlpha
+
+        Pass
+        {
+            Name "RiveGammaDecodeUI"
+
+            CGPROGRAM
+            #pragma vertex vert
+            #pragma fragment frag
+            #pragma target 2.0
+
+            #include "UnityCG.cginc"
+            #include "UnityUI.cginc"
+
+            #pragma multi_compile __ UNITY_UI_CLIP_RECT
+            #pragma multi_compile __ UNITY_UI_ALPHACLIP
+
+            struct appdata_t
+            {
+                float4 vertex   : POSITION;
+                float4 color    : COLOR;
+                float2 texcoord : TEXCOORD0;
+                float2 texcoord1 : TEXCOORD1; // used for RectMask2D
+                UNITY_VERTEX_INPUT_INSTANCE_ID
+            };
+
+            struct v2f
+            {
+                float4 vertex        : SV_POSITION;
+                fixed4 color         : COLOR;
+                float2 texcoord      : TEXCOORD0;
+                float4 worldPosition : TEXCOORD1;
+                UNITY_VERTEX_OUTPUT_STEREO
+            };
+
+            sampler2D _MainTex;
+            float4 _MainTex_ST;
+            fixed4 _Color;
+            float4 _ClipRect;
+
+            v2f vert (appdata_t v)
+            {
+                v2f o;
+                UNITY_SETUP_INSTANCE_ID(v);
+                UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);
+
+                o.worldPosition = v.vertex;
+                o.vertex = UnityObjectToClipPos(v.vertex);
+                o.texcoord = TRANSFORM_TEX(v.texcoord, _MainTex);
+                o.color = v.color * _Color;
+                return o;
+            }
+
+            half4 frag (v2f i) : SV_Target
+            {
+                // Sample texture
+                half4 tex = tex2D(_MainTex, i.texcoord);
+
+                // Rive renders premultiplied alpha into the render texture.
+                // Convert to linear in un-premultiplied space, then premultiply again.
+                #if !defined(UNITY_COLORSPACE_GAMMA)
+                    if (tex.a > 0.0h)
+                    {
+                        half3 unpremultiplied = tex.rgb / tex.a;
+                        tex.rgb = GammaToLinearSpace(unpremultiplied) * tex.a;
+                    }
+                #endif
+
+                // Preserve premultiplied alpha when Unity UI applies tint/CanvasGroup alpha.
+                half4 color;
+                color.a = tex.a * i.color.a;
+                color.rgb = tex.rgb * i.color.rgb * i.color.a;
+
+                // RectMask2D clipping
+                // we scale both RGB and A to keep premultiplied invariant
+                #ifdef UNITY_UI_CLIP_RECT
+                    half clipFactor = UnityGet2DClipping(i.worldPosition.xy, _ClipRect);
+                    color *= clipFactor;
+                #endif
+
+                // Optional alpha clipping for masking
+                #ifdef UNITY_UI_ALPHACLIP
+                    clip (color.a - 0.001);
+                #endif
+
+                return color;
+            }
+            ENDCG
+        }
+    }
+
+    Fallback "UI/Default"
+}