cc: don't allow transparent quads for opaque layers

This is a DCHECK + workaround for issue 988590.

https://chromium-review.googlesource.com/c/chromium/src/+/1749703
solves root of the problem, and will avoid this DCHECK for
this particular content.

Bug: 988590
Change-Id: I0e66ba388c11bbc3805ead42343d7fe7af7a79da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752873
Commit-Queue: enne <enne@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689250}

cc/debug/debug_colors.cc

@@ -128,6 +128,11 @@ int DebugColors::OOMTileBorderWidth(float device_scale_factor) {
   return Scale(1, device_scale_factor);
 }
 
+// Missing opaque colors are magenta.
+SkColor DebugColors::MissingOpaqueTileColor() {
+  return SK_ColorMAGENTA;
+}
+
 // Direct picture borders are chartreuse.
 SkColor DebugColors::DirectPictureBorderColor() {
   return SkColorSetARGB(255, 127, 255, 0);

cc/debug/debug_colors.h

@@ -57,6 +57,8 @@ class CC_DEBUG_EXPORT DebugColors {
   static SkColor OOMTileBorderColor();
   static int OOMTileBorderWidth(float device_scale_factor);
 
+  static SkColor MissingOpaqueTileColor();
+
   static SkColor DirectPictureBorderColor();
   static int DirectPictureBorderWidth(float device_scale_factor);
 

cc/layers/picture_layer_impl.cc

@@ -232,11 +232,30 @@ void PictureLayerImpl::AppendQuads(viz::RenderPass* render_pass,
       occlusion = draw_properties().occlusion_in_content_space;
     }
 
+    SkColor color = raster_source_->GetSolidColor();
+    if (contents_opaque() && SkColorGetA(color) != 255) {
+      // If we get to this code, then this has been a failure of
+      // solid color detection on a layer that we think should
+      // be opaque.
+      //
+      // It would be nice to NOTREACHED() this, but too many
+      // ui/ tests set up transparent-but-opaque layers.
+      //
+      // Because this could lead to spoofing between pages, and
+      // other severe graphical issues due to not clearing the
+      // backbuffer, also set this color to something known to be
+      // opaque.
+#ifndef NDEBUG
+      color = DebugColors::MissingOpaqueTileColor();
+#else
+      color = SafeOpaqueBackgroundColor();
+#endif
+    }
+
     EffectNode* effect_node = GetEffectTree().Node(effect_tree_index());
     SolidColorLayerImpl::AppendSolidQuads(
         render_pass, occlusion, shared_quad_state, scaled_visible_layer_rect,
-        raster_source_->GetSolidColor(),
-        !layer_tree_impl()->settings().enable_edge_anti_aliasing,
+        color, !layer_tree_impl()->settings().enable_edge_anti_aliasing,
         effect_node->blend_mode, append_quads_data);
     return;
   }
@@ -441,16 +460,34 @@ void PictureLayerImpl::AppendQuads(viz::RenderPass* render_pass,
           break;
         }
         case TileDrawInfo::SOLID_COLOR_MODE: {
-          float alpha =
-              (SkColorGetA(draw_info.solid_color()) * (1.0f / 255.0f)) *
-              shared_quad_state->opacity;
+          SkColor color = draw_info.solid_color();
+          if (contents_opaque() && SkColorGetA(color) != 255) {
+            // If we get to this code, then this has been a failure of
+            // solid color detection on a layer that we think should
+            // be opaque.
+            //
+            // It would be nice to NOTREACHED() this, but too many
+            // ui/ tests set up transparent-but-opaque layers.
+            //
+            // Because this could lead to spoofing between pages, and
+            // other severe graphical issues due to not clearing the
+            // backbuffer, also set this color to something known to be
+            // opaque.
+#ifndef NDEBUG
+            color = DebugColors::MissingOpaqueTileColor();
+#else
+            color = SafeOpaqueBackgroundColor();
+#endif
+          }
+          float alpha = (SkColorGetA(color) * (1.0f / 255.0f)) *
+                        shared_quad_state->opacity;
           if (mask_type_ != Layer::LayerMaskType::NOT_MASK ||
               alpha >= std::numeric_limits<float>::epsilon()) {
             auto* quad =
                 render_pass->CreateAndAppendDrawQuad<viz::SolidColorDrawQuad>();
             quad->SetNew(
                 shared_quad_state, offset_geometry_rect,
-                offset_visible_geometry_rect, draw_info.solid_color(),
+                offset_visible_geometry_rect, color,
                 !layer_tree_impl()->settings().enable_edge_anti_aliasing);
             ValidateQuadResources(quad);
           }