Bug 1567472 - Fix bug in preserve-3d batching code in WebRender. r=nical

The code to batch preserve-3d elements was incorrectly using the
bounds and visibility mask from the parent element. This could
result in batching bugs in some cases, which were showing up as
draw order issues.

Differential Revision: https://phabricator.services.mozilla.com/D38834

--HG--
extra : moz-landing-system : lando

gfx/wr/webrender/src/batch.rs

@@ -1137,6 +1137,10 @@ impl BatchBuilder {
                                 gpu_cache,
                             );
 
+                            // Need a new z-id for each child preserve-3d context added
+                            // by this inner loop.
+                            let z_id = z_generator.next();
+
                             let prim_header_index = prim_headers.push(&prim_header, z_id, [
                                 uv_rect_address.as_int(),
                                 if raster_config.establishes_raster_root { 1 } else { 0 },
@@ -1152,11 +1156,11 @@ impl BatchBuilder {
 
                             self.add_split_composite_instance_to_batches(
                                 key,
-                                &prim_info.clip_chain.pic_clip_rect,
+                                &child_prim_info.clip_chain.pic_clip_rect,
                                 z_id,
                                 prim_header_index,
                                 child.gpu_address,
-                                prim_info.visibility_mask,
+                                child_prim_info.visibility_mask,
                             );
                         }
                     }

gfx/wr/wrench/reftests/split/order-3-ref.yaml

@@ -0,0 +1,15 @@
+---
+root:
+  items:
+    -
+      type: rect
+      bounds: [8, 8, 200, 200]
+      color: blue
+    -
+      type: rect
+      bounds: [8, 208, 200, 200]
+      color: blue
+    -
+      type: rect
+      bounds: [8, 408, 200, 200]
+      color: blue

gfx/wr/wrench/reftests/split/order-3.yaml

@@ -0,0 +1,31 @@
+# Ensure that preserve-3d items are correctly batched with transparent siblings.
+---
+root:
+  items:
+    -
+      type: stacking-context
+      transform-style: "preserve-3d"
+      items:
+        -
+          type: rect
+          bounds: [8, 8, 200, 200]
+          color: blue
+    -
+      type: stacking-context
+      transform-style: "preserve-3d"
+      items:
+        -
+          type: rect
+          bounds: [8, 208, 200, 200]
+          color: blue
+    - type: rect
+      color: [255, 0, 0, 0.5]
+      bounds: [8, 408, 200, 200]
+    -
+      type: stacking-context
+      transform-style: "preserve-3d"
+      items:
+        -
+          type: rect
+          bounds: [8, 408, 200, 200]
+          color: blue

gfx/wr/wrench/reftests/split/reftest.list

@@ -19,3 +19,4 @@ platform(linux,mac) fuzzy(1,20) == same-plane.yaml same-plane.png
 == mixed-order.yaml mixed-order-ref.yaml
 fuzzy(1,40000) == filter.yaml filter-ref.yaml
 fuzzy(1,10000) == gradient.yaml gradient-ref.yaml
+== order-3.yaml order-3-ref.yaml