If a stacking context is isolated, disable subpixel AA on it.

This is a workaround / hack for #1839, until we properly support
subpixel AA on transparent surfaces. It extends the set of
conditions that disable subpixel AA on a stacking context.

Fixes #1839.

webrender/src/frame_builder.rs

@@ -1153,7 +1153,7 @@ impl FrameBuilder {
             //           we handle subpixel blending.
             if let Some(sc_index) = self.stacking_context_stack.last() {
                 let stacking_context = &self.stacking_context_store[sc_index.0];
-                if stacking_context.composite_ops.count() > 0 {
+                if !stacking_context.allow_subpixel_aa {
                     render_mode = FontRenderMode::Alpha;
                 }
             }

webrender/src/tiling.rs

@@ -1601,6 +1601,11 @@ pub struct StackingContext {
 
     /// Current stacking context visibility of backface.
     pub is_backface_visible: bool,
+
+    /// Allow subpixel AA for text runs on this stacking context.
+    /// This is a temporary hack while we don't support subpixel AA
+    /// on transparent stacking contexts.
+    pub allow_subpixel_aa: bool,
 }
 
 impl StackingContext {
@@ -1618,6 +1623,8 @@ impl StackingContext {
             TransformStyle::Flat => ContextIsolation::None,
             TransformStyle::Preserve3D => ContextIsolation::Items,
         };
+        let allow_subpixel_aa = composite_ops.count() == 0 &&
+                                isolation == ContextIsolation::None;
         StackingContext {
             pipeline_id,
             reference_frame_offset,
@@ -1630,6 +1637,7 @@ impl StackingContext {
             is_pipeline_root,
             is_visible: false,
             is_backface_visible,
+            allow_subpixel_aa,
         }
     }
 

wrench/reftests/text/isolated-text.yaml

@@ -0,0 +1,14 @@
+root:
+  items:
+    - type: stacking-context
+      bounds: [0, 0, 1000, 100]
+      transform: perspective(1000)
+      items:
+      - type: stacking-context
+        bounds: [0, 0, 1000, 100]
+        transform-style: preserve-3d
+        items:
+          - text: "Cats making all the muffins knock over christmas tree"
+            origin: 20 30
+            size: 20
+            font: "FreeSans.ttf"

wrench/reftests/text/reftest.list

@@ -23,3 +23,5 @@ fuzzy(1,160) == shadow-grey.yaml shadow-grey-ref.yaml
 # enable when synthetic-italics implemented on mac
 platform(linux) != synthetic-italics.yaml synthetic-italics-ref.yaml
 options(disable-subpixel) == ahem.yaml ahem-ref.yaml
+platform(linux) == isolated-text.yaml isolated-text.png
+

wrench/src/yaml_helper.rs

@@ -332,6 +332,9 @@ impl YamlHelper for Yaml {
                         "rotate-y" if args.len() == 1 => {
                             make_rotation(transform_origin, args[0].parse().unwrap(), 0.0, 1.0, 0.0)
                         }
+                        "perspective" if args.len() == 1 => {
+                            LayoutTransform::create_perspective(args[0].parse().unwrap())
+                        }
                         _ => {
                             println!("unknown function {}", function);
                             break;