Correctly handle blend primitives, where the contents are clipped.

webrender/res/brush_blend.glsl

@@ -13,6 +13,7 @@ flat varying float vAmount;
 flat varying int vOp;
 flat varying mat4 vColorMat;
 flat varying vec4 vColorOffset;
+flat varying vec4 vUvClipBounds;
 
 #ifdef WR_VERTEX_SHADER
 
@@ -30,6 +31,10 @@ void brush_vs(
               src_task.content_origin;
     vUv = vec3(uv / texture_size, src_task.common_data.texture_layer_index);
 
+    vec2 uv0 = src_task.common_data.task_rect.p0;
+    vec2 uv1 = uv0 + src_task.common_data.task_rect.size;
+    vUvClipBounds = vec4(uv0, uv1) / texture_size.xyxy;
+
     vOp = user_data.y;
 
     float lumR = 0.2126;
@@ -142,6 +147,10 @@ vec4 brush_fs() {
             color = vColorMat * Cs + vColorOffset;
     }
 
+    // Fail-safe to ensure that we don't sample outside the rendered
+    // portion of a blend source.
+    color.a *= point_inside_rect(vUv.xy, vUvClipBounds.xy, vUvClipBounds.zw);
+
     // Pre-multiply the alpha into the output value.
     color.rgb *= color.a;
 

webrender/res/rect.glsl

@@ -46,3 +46,8 @@ bool rect_inside_rect(RectWithSize little, RectWithSize big) {
     return all(lessThanEqual(vec4(big.p0, little.p0 + little.size),
                              vec4(little.p0, big.p0 + big.size)));
 }
+
+float point_inside_rect(vec2 p, vec2 p0, vec2 p1) {
+    vec2 s = step(p0, p) - step(p1, p);
+    return s.x * s.y;
+}

webrender/src/batch.rs

@@ -301,12 +301,19 @@ impl BatchList {
         }
     }
 
+    fn add_bounding_rect(
+        &mut self,
+        task_relative_bounding_rect: &DeviceIntRect,
+    ) {
+        self.combined_bounding_rect = self.combined_bounding_rect.union(task_relative_bounding_rect);
+    }
+
     pub fn get_suitable_batch(
         &mut self,
         key: BatchKey,
         task_relative_bounding_rect: &DeviceIntRect,
     ) -> &mut Vec<PrimitiveInstance> {
-        self.combined_bounding_rect = self.combined_bounding_rect.union(task_relative_bounding_rect);
+        self.add_bounding_rect(task_relative_bounding_rect);
 
         match key.blend_mode {
             BlendMode::None => {
@@ -1242,6 +1249,8 @@ impl AlphaBatchBuilder {
             user_data,
         };
 
+        self.batch_list.add_bounding_rect(task_relative_bounding_rect);
+
         match brush.segment_desc {
             Some(ref segment_desc) => {
                 let alpha_batch_key = BatchKey {

wrench/reftests/filters/blend-clipped.yaml

@@ -0,0 +1,110 @@
+---
+root:
+  items:
+    -
+      type: "stacking-context"
+      items:
+        -
+          bounds: [0, 0, 1887, 2081]
+          "clip-rect": [0, 0, 1887, 2081]
+          "clip-and-scroll": 0
+          "backface-visible": true
+          type: clip
+          id: 1
+          "content-size": [1887, 2081]
+        -
+          bounds: [0, 111, 1887, 1970]
+          "clip-rect": [0, 111, 1887, 1970]
+          "clip-and-scroll": 0
+          "backface-visible": true
+          type: clip
+          id: 2
+          "content-size": [1887, 1970]
+        -
+          bounds: [0, 111, 1887, 1971]
+          "clip-rect": [0, 111, 1887, 1971]
+          "clip-and-scroll": 2
+          "backface-visible": true
+          type: iframe
+          id: [1, 3]
+  id: [1, 1]
+pipelines:
+  -
+    id: [1, 3]
+    items:
+      -
+        type: "stacking-context"
+        items:
+          -
+            bounds: [0, 0, 1887, 1971]
+            "clip-rect": [0, 0, 1887, 1971]
+            "clip-and-scroll": 0
+            type: clip
+            id: 1
+            "content-size": [1887, 1971]
+          -
+            "clip-rect": [0, 0, 1887, 1971]
+            "clip-and-scroll": 1
+            type: "scroll-frame"
+            id: 2
+            "content-size": [1887, 1971]
+            bounds: [0, 0, 1887, 1971]
+          -
+            bounds: [0, 0, 1887, 1971]
+            "clip-rect": [0, 0, 1887, 1971]
+            "clip-and-scroll": 2
+            type: clip
+            id: 3
+            "content-size": [1887, 1971]
+          -
+            "clip-and-scroll": 3
+            type: "stacking-context"
+            items:
+              -
+                bounds: [0, -186, 1887, 239]
+                "clip-rect": [0, -186, 1887, 239]
+                "clip-and-scroll": 3
+                "backface-visible": true
+                type: clip
+                id: 4
+                "content-size": [1887, 239]
+              -
+                bounds: [-660.45, -186, 0, 0]
+                "clip-rect": [-660.45, -186, 0, 0]
+                "clip-and-scroll": 4
+                type: "stacking-context"
+                transform: [1, 0, 0, 0, -0.57735026, 1, 0, 0, 0, 0, 1, 0, 68.849, 0, 0, 1]
+                items:
+                  -
+                    "clip-and-scroll": 4
+                    type: "stacking-context"
+                    filters:
+                      - opacity(0.8)
+                    items:
+                      -
+                        bounds: [0, 0, 1887, 239]
+                        "clip-rect": [0, 0, 1887, 239]
+                        "clip-and-scroll": 4
+                        type: clip
+                        id: 5
+                        "content-size": [1887, 239]
+                      -
+                        bounds: [0, 0, 1887, 239]
+                        "clip-rect": [0, 0, 1887, 239]
+                        "clip-and-scroll": 5
+                        "backface-visible": true
+                        type: gradient
+                        start: [943.5, -0.00000000000005684342]
+                        end: [943.5, 238.5]
+                        "tile-size": [1887, 238.5]
+                        "tile-spacing": [0, 0]
+                        stops:
+                          - 0
+                          - 0 153 204.00002 1.0000
+                          - 0.3
+                          - 0 153 204.00002 1.0000
+                          - 0.6
+                          - 7.0000005 137 194.00002 1.0000
+                          - 1
+                          - 0 91 137 1.0000
+                        repeat: false

wrench/reftests/filters/reftest.list

@@ -32,3 +32,5 @@
 == filter-long-chain.yaml filter-long-chain.png
 == filter-drop-shadow.yaml filter-drop-shadow.png
 == filter-drop-shadow-on-viewport-edge.yaml filter-drop-shadow-on-viewport-edge.png
+== blend-clipped.yaml blend-clipped.png
+