Bug 1368551 - Don't send transforms to WR in push_stacking_context if they are identity. r=pchang

If we set a transform in push_stacking_context, it changes the internal
WebRender behaviour to make that stacking context a reference frame, and
things inside it are positioned differently. This is true even if the
transform is an identity transform.

In most cases we are hitting this and sending an identity transform
through, when in fact we want to be sending a None value to WebRender so
that it doesn't create reference frames. This is a partial fix, a proper
fix will be done in bug 1345577 by separating the CSS transform from the
other transforms that FrameLayerBuilder invents.

MozReview-Commit-ID: ElSs3hFMD2D

gfx/layers/wr/StackingContextHelper.cpp

@@ -27,7 +27,8 @@ StackingContextHelper::StackingContextHelper(const StackingContextHelper& aParen
   Layer* layer = aLayer->GetLayer();
   mTransform = aTransform.valueOr(layer->GetTransform());
   float opacity = 1.0f;
-  mBuilder->PushStackingContext(scBounds, 0, &opacity, &mTransform,
+  mBuilder->PushStackingContext(scBounds, 0, &opacity,
+                                mTransform.IsIdentity() ? nullptr : &mTransform,
                                 wr::ToWrMixBlendMode(layer->GetMixBlendMode()));
   mOrigin = aLayer->Bounds().TopLeft();
 }

gfx/layers/wr/WebRenderContainerLayer.cpp

@@ -107,6 +107,13 @@ WebRenderContainerLayer::RenderLayer(wr::DisplayListBuilder& aBuilder,
     transformForSC = nullptr;
   }
 
+  if (transformForSC && transform.IsIdentity()) {
+    // If the transform is an identity transform, strip it out so that WR
+    // doesn't turn this stacking context into a reference frame, as it
+    // affects positioning. Bug 1345577 tracks a better fix.
+    transformForSC = nullptr;
+  }
+
   ScrollingLayersHelper scroller(this, aBuilder, aSc);
   StackingContextHelper sc(aSc, aBuilder, this, animationsId, opacityForSC, transformForSC);
 

gfx/webrender_bindings/src/bindings.rs

@@ -1261,16 +1261,19 @@ pub extern "C" fn wr_dp_push_stacking_context(state: &mut WrState,
     }
 
     let transform = unsafe { transform.as_ref() };
-    let transform_binding = match transform {
-        Some(transform) => PropertyBinding::Value(transform.into()),
-        None => PropertyBinding::Binding(PropertyBindingKey::new(animation_id)),
+    let transform_binding = match animation_id {
+        0 => match transform {
+            Some(transform) => Some(PropertyBinding::Value(transform.into())),
+            None => None,
+        },
+        _ => Some(PropertyBinding::Binding(PropertyBindingKey::new(animation_id))),
     };
 
     state.frame_builder
          .dl_builder
          .push_stacking_context(webrender_traits::ScrollPolicy::Scrollable,
                                 bounds,
-                                Some(transform_binding),
+                                transform_binding,
                                 TransformStyle::Flat,
                                 None,
                                 mix_blend_mode,