Correct output clip of scrollbar effect nodes

The output clip should be the parent of the current clip (the overflow
clip). Scrollbars are not clipped by the overflow clip. This didn't
cause any visible problem because we ignore the clip when we see the
PaintChunk of the scrollbar escapes the current effect's output clip,
but that caused annoying DLOG(ERROR) in PaintChunksToCcLayer for
in abnormal situation.

The incorrect output clip was added in crrev.com/1164980.

Bug: 1422877
Change-Id: I3927fa34ef8120ae9725bda57d72ddb839cf897e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4939807
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1210369}

third_party/blink/renderer/core/paint/paint_property_tree_builder.cc

@@ -2568,7 +2568,14 @@ void FragmentPaintPropertyTreeBuilder::UpdateScrollAndScrollTranslation() {
             if (needs_effect_node) {
               EffectPaintPropertyNode::State effect_state;
               effect_state.local_transform_space = context_.current.transform;
-              effect_state.output_clip = context_.current.clip;
+              if (properties_->OverflowClip()) {
+                // Scrollbars are not clipped by OverflowClip, so the output
+                // clip should be the parent of the overflow clip.
+                DCHECK_EQ(context_.current.clip, properties_->OverflowClip());
+                effect_state.output_clip = context_.current.clip->Parent();
+              } else {
+                effect_state.output_clip = context_.current.clip;
+              }
               effect_state.compositor_element_id =
                   scrollable_area->GetScrollbarElementId(orientation);
 

third_party/blink/renderer/core/paint/paint_property_tree_builder_test.cc

@@ -7286,4 +7286,21 @@ TEST_P(PaintPropertyTreeBuilderTest, BackgroundClipFragmented) {
                          gfx::RectF(200, 0, 100, 200));
 }
 
+TEST_P(PaintPropertyTreeBuilderTest, OverlayScrollbarEffects) {
+  SetBodyInnerHTML(R"HTML(
+    <div id="target" style="width: 100px; height: 100px; overflow: scroll">
+      <div style="height: 300px"></div>
+    </div>
+  )HTML");
+  CHECK(GetDocument().GetPage()->GetScrollbarTheme().UsesOverlayScrollbars());
+
+  auto* properties = PaintPropertiesForElement("target");
+  ASSERT_TRUE(properties);
+  ASSERT_TRUE(properties->OverflowClip());
+  EXPECT_FALSE(properties->HorizontalScrollbarEffect());
+  ASSERT_TRUE(properties->VerticalScrollbarEffect());
+  EXPECT_EQ(properties->OverflowClip()->Parent(),
+            properties->VerticalScrollbarEffect()->OutputClip());
+}
+
 }  // namespace blink