Add extra DCHECKing to try to catch subpixel rendering on a non-opaque

ui/views/controls/label.cc

Index: ui/views/controls/label.cc
diff --git a/ui/views/controls/label.cc b/ui/views/controls/label.cc
index 2086222e63f3c879d426a3d4509f7f2ec612a0d8..6b71eb17965db42ae7cebd9cbd080d241b663408 100644
--- a/ui/views/controls/label.cc
+++ b/ui/views/controls/label.cc
@@ -450,8 +450,27 @@ std::unique_ptr<gfx::RenderText> Label::CreateRenderText(
 
 void Label::PaintText(gfx::Canvas* canvas) {
   MaybeBuildRenderTextLines();
-  for (size_t i = 0; i < lines_.size(); ++i)
+
+  for (size_t i = 0; i < lines_.size(); ++i) {
     lines_[i]->Draw(canvas);
+#if DCHECK_IS_ON()
+    // Attempt to ensure that if we're using subpixel rendering, we're painting
+    // to an opaque background. What we don't want to find is an ancestor in the
+    // hierarchy that paints to a non-opaque layer.
+    if (!lines_[i]->subpixel_rendering_suppressed()) {

[Daniel Erat, 2017/02/06 22:35:49]

as a minor optimization, you can skip the stuff inside of this if block after
the first non-suppressed line that you find, right? right now, it looks like you
can check the hierarchy multiple times.

[Evan Stade, 2017/02/07 23:55:19]

yea, we can just do this for the first line since all the lines should have the
same value for this variable.

+      for (View* view = this; view; view = view->parent()) {
+        if (view->background())
+          break;

[sadrul, 2017/02/07 03:36:39]

The background could be non-opaque (e.g. solid-color background with alpha < 1),
and live in a non-opaque layer too, right? So the existence of a background
doesn't necessarily give you what you want.

[Evan Stade, 2017/02/07 23:51:23]

Correct, that's the situation I was referring to in the CL description that
wouldn't be caught, i.e. would be a false negative.

+        if (view->layer()) {
+          DCHECK(view->layer()->fills_bounds_opaquely())
+              << " Ancestor view has a non-opaque layer: "
+              << view->GetClassName() << " with ID " << view->id();
+          break;
+        }
+      }
+    }
+#endif
+  }
 }
 
 void Label::OnBoundsChanged(const gfx::Rect& previous_bounds) {
@@ -964,7 +983,8 @@ void Label::RecalculateColors() {
 void Label::ApplyTextColors() const {
   SkColor color = enabled() ? actual_enabled_color_ : actual_disabled_color_;
   bool subpixel_rendering_suppressed =
-      SkColorGetA(background_color_) != 0xFF || !subpixel_rendering_enabled_;
+      SkColorGetA(background_color_) != SK_AlphaOPAQUE ||
+      !subpixel_rendering_enabled_;
   for (size_t i = 0; i < lines_.size(); ++i) {
     lines_[i]->SetColor(color);
     lines_[i]->set_selection_color(actual_selection_text_color_);