Fix bug in get_relative_transform.

Fixes the maze solver bug (bugzilla 1487984).

webrender/src/clip_scroll_tree.rs

@@ -148,7 +148,7 @@ impl ClipScrollTree {
                                  .pre_mul(&coord_system.transform);
         }
 
-        let transform = transform.post_translate(
+        let transform = transform.pre_translate(
             LayoutVector3D::new(
                 child.coordinate_system_relative_offset.x,
                 child.coordinate_system_relative_offset.y,
@@ -649,8 +649,8 @@ fn test_cst_scale_translation() {
     test_pt(100.0, 100.0, &cst, child2, child1, 200.0, 400.0);
     test_pt(200.0, 400.0, &cst, child1, child2, 100.0, 100.0);
 
-    test_pt(100.0, 100.0, &cst, child3, child1, 400.0, 300.0);
-    test_pt(400.0, 300.0, &cst, child1, child3, 100.0, 100.0);
+    test_pt(100.0, 100.0, &cst, child3, child1, 600.0, 0.0);
+    test_pt(400.0, 300.0, &cst, child1, child3, 0.0, 175.0);
 }
 
 #[test]

wrench/reftests/transforms/clip-translate-ref.yaml

@@ -0,0 +1,6 @@
+---
+root:
+  items:
+    - type: rect
+      color: red
+      bounds: [6, 168, 18, 18]

wrench/reftests/transforms/clip-translate.yaml

@@ -0,0 +1,21 @@
+---
+root:
+  items:
+    -
+      type: "stacking-context"
+      transform: [0.75, 0, 0, 0, 0, 0.75, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1]
+      items:
+        -
+          type: "stacking-context"
+          transform: [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 8, 224, 0, 1]
+          items:
+            - type: clip
+              bounds: [0, 0, 24, 24]
+              image-mask:
+                image: solid-color(255, 0, 0, 255, 24, 24)
+                rect: [0, 0, 24, 24]
+                repeat: false
+              items:
+                - type: rect
+                  color: red
+                  bounds: [0, 0, 24, 24]

wrench/reftests/transforms/reftest.list

@@ -21,3 +21,4 @@ platform(linux,mac) == nested-preserve-3d.yaml nested-preserve-3d.png
 platform(linux,mac) == near-plane-clip.yaml near-plane-clip.png
 platform(linux,mac) == perspective-mask.yaml perspective-mask.png
 rotate-clip.yaml rotate-clip-ref.yaml
+clip-translate.yaml clip-translate-ref.yaml