Handle position:absolute candidates correctly in ScrollAnchor.

An absolute-positioned candidate should be considered only if it would
actually scroll with the scroller (otherwise the anchoring scroll will have no
effect on it).

BUG=594873

Review URL: https://codereview.chromium.org/1831993002

Cr-Commit-Position: refs/heads/master@{#383361}

2 files changed, 51 insertions, 1 deletions

diff --git a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
index ba2558b..7959b41 100644
--- a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
+++ b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
@@ -86,6 +86,16 @@ static LayoutPoint computeRelativeOffset(const LayoutObject* layoutObject, const
     return cornerPointOfRect(relativeBounds(layoutObject, scroller), corner);
 }
 
+static bool candidateMovesWithScroller(const LayoutObject* candidate, const ScrollableArea* scroller)
+{
+    if (candidate->style() && candidate->style()->hasViewportConstrainedPosition())
+        return false;
+
+    bool skippedByContainerLookup = false;
+    candidate->container(scrollerLayoutBox(scroller), &skippedByContainerLookup);
+    return !skippedByContainerLookup;
+}
+
 ScrollAnchor::ExamineResult ScrollAnchor::examine(const LayoutObject* candidate) const
 {
     if (candidate->isLayoutInline())
@@ -94,7 +104,7 @@ ScrollAnchor::ExamineResult ScrollAnchor::examine(const LayoutObject* candidate)
     if (!candidate->isText() && !candidate->isBox())
         return ExamineResult(Skip);
 
-    if (candidate->style() && candidate->style()->hasViewportConstrainedPosition())
+    if (!candidateMovesWithScroller(candidate, m_scroller))
         return ExamineResult(Skip);
 
     LayoutRect candidateRect = relativeBounds(candidate, m_scroller);
diff --git a/third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp b/third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp
index 0f7a5e0..f52e831 100644
--- a/third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp
+++ b/third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp
@@ -180,6 +180,46 @@ TEST_F(ScrollAnchorTest, ExcludeFixedPosition)
         scrollAnchor(layoutViewport()).anchorObject());
 }
 
+TEST_F(ScrollAnchorTest, ExcludeAbsolutePosition)
+{
+    setBodyInnerHTML(
+        "<style>"
+        "    body { margin: 0; }"
+        "    #scroller { overflow: scroll; width: 500px; height: 400px; }"
+        "    #space { height: 1000px; }"
+        "    #abs {"
+        "        position: absolute; background-color: red;"
+        "        left: 200px; top: 100px; width: 100px; height: 100px;"
+        "    }"
+        "    #rel {"
+        "        position: relative; background-color: green;"
+        "        left: 50px; top: 100px; width: 100px; height: 75px;"
+        "    }"
+        "</style>"
+        "<div id='scroller'><div id='space'>"
+        "    <div id='abs'></div>"
+        "    <div id='rel'></div>"
+        "</div></div>");
+
+    Element* scrollerElement = document().getElementById("scroller");
+    ScrollableArea* scroller = scrollerForElement(scrollerElement);
+    Element* absPos = document().getElementById("abs");
+    Element* relPos = document().getElementById("rel");
+
+    scroller->scrollBy(DoubleSize(0, 25), UserScroll);
+    setHeight(relPos, 100);
+
+    // When the scroller is position:static, the anchor cannot be position:absolute.
+    EXPECT_EQ(relPos->layoutObject(), scrollAnchor(scroller).anchorObject());
+
+    scrollerElement->setAttribute(HTMLNames::styleAttr, "position: relative");
+    scroller->scrollBy(DoubleSize(0, 25), UserScroll);
+    setHeight(relPos, 125);
+
+    // When the scroller is position:relative, the anchor may be position:absolute.
+    EXPECT_EQ(absPos->layoutObject(), scrollAnchor(scroller).anchorObject());
+}
+
 class ScrollAnchorCornerTest : public ScrollAnchorTest {
 protected:
     void checkCorner(const AtomicString& id, Corner corner, DoublePoint startPos, DoubleSize expectedAdjustment)