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

ui/views/controls/label.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/controls/label.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <cmath>
@@ skipping 432 matching lines @@
   render_text->SetMinLineHeight(line_height());
   render_text->SetFontList(font_list());
   render_text->set_shadows(shadows());
   render_text->SetCursorEnabled(false);
   render_text->SetText(text);
   return render_text;
 }
 
 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) {
   if (previous_bounds.size() != size())
     InvalidateLayout();
 }
 
 void Label::OnPaint(gfx::Canvas* canvas) {
   View::OnPaint(canvas);
   if (is_first_paint_text_) {
@@ skipping 492 matching lines @@
                                           selection_background_color_)
           : requested_selection_text_color_;
 
   ApplyTextColors();
   SchedulePaint();
 }
 
 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_);
     lines_[i]->set_selection_background_focused_color(
         selection_background_color_);
     lines_[i]->set_subpixel_rendering_suppressed(subpixel_rendering_suppressed);
   }
 }
 
 void Label::UpdateColorsFromTheme(const ui::NativeTheme* theme) {
@@ skipping 54 matching lines @@
       .WriteText(GetSelectedText());
 }
 
 void Label::BuildContextMenuContents() {
   context_menu_contents_.AddItemWithStringId(IDS_APP_COPY, IDS_APP_COPY);
   context_menu_contents_.AddItemWithStringId(IDS_APP_SELECT_ALL,
                                              IDS_APP_SELECT_ALL);
 }
 
 }  // namespace views