Don't apply composited scrolling offsets if composited scrolling isn't happening.

BUG=646363 CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2 Review-Url: https://codereview.chromium.org/2343963002 Cr-Commit-Position: refs/heads/master@{#418965}

Don't apply composited scrolling offsets if composited scrolling isn't happening.
BUG=646363 CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2 Review-Url: https://codereview.chromium.org/2343963002 Cr-Commit-Position: refs/heads/master@{#418965}
57823749 · chrishtr · Commit bot · cbf3eb05 · 57823749 · 57823749
Commit 57823749 authored Sep 15, 2016 by chrishtr Committed by Commit bot Sep 15, 2016
4 changed files
--- a/third_party/WebKit/LayoutTests/compositing/overflow/scroll-neg-z-index-and-composited-child-expected.png
+++ b/third_party/WebKit/LayoutTests/compositing/overflow/scroll-neg-z-index-and-composited-child-expected.png
--- a/third_party/WebKit/LayoutTests/compositing/overflow/scroll-neg-z-index-and-composited-child-expected.txt
+++ b/third_party/WebKit/LayoutTests/compositing/overflow/scroll-neg-z-index-and-composited-child-expected.txt
+layer at (0,0) size 800x600
+  LayoutView at (0,0) size 800x600
+layer at (0,0) size 800x300
+  LayoutBlockFlow {HTML} at (0,0) size 800x300
+    LayoutBlockFlow {BODY} at (0,0) size 800x300
+layer at (0,0) size 800x300 scrollY 50.00 scrollWidth 1000 scrollHeight 1050 layerType: background only
+layer at (0,950) size 800x0
+  LayoutBlockFlow (relative positioned) zI: -1 {DIV} at (0,1000) size 800x0
+layer at (0,0) size 800x300 scrollY 50.00 scrollWidth 1000 scrollHeight 1050 layerType: foreground only
+  LayoutBlockFlow (relative positioned) {DIV} at (0,0) size 800x300 [bgcolor=#808080]
+layer at (0,0) size 1000x1000 backgroundClip at (0,0) size 800x300 clip at (0,0) size 800x300
+  LayoutBlockFlow (relative positioned) {DIV} at (0,0) size 1000x1000 [bgcolor=#FFFFFF]
--- a/third_party/WebKit/LayoutTests/compositing/overflow/scroll-neg-z-index-and-composited-child.html
+++ b/third_party/WebKit/LayoutTests/compositing/overflow/scroll-neg-z-index-and-composited-child.html
+<!doctype HTML>
+<style>
+body {
+  margin: 0;
+}
+::-webkit-scrollbar {
+  display: none;
+}
+#scroller {
+  background: gray;
+  height: 300px;
+  overflow: scroll;
+  position: relative;
+  z-index: 0;
+}
+
+#content {
+  overflow: hidden;
+  position: relative;
+  background-color: white;
+  width: 1000px;
+  height: 1000px;
+  top: 50px;
+}
+</style>
+<!-- The test succeeds if white is visible across all of the visible scrolling area, and no gray -->
+<div id="scroller">
+  <div id="content"></div>
+  <div style="position: relative; z-index: -1; will-change: transform"></div>
+</div>
+<script>
+onload = function() {
+  scroller.scrollTop = 50;
+}
+</script>
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -2434,10 +2434,12 @@ void CompositedLayerMapping::adjustForCompositedScrolling(const GraphicsLayer* g
 {
    if (graphicsLayer == m_scrollingContentsLayer.get() || graphicsLayer == m_foregroundLayer.get()) {
        if (PaintLayerScrollableArea* scrollableArea = m_owningLayer.getScrollableArea()) {
-            // Note: this is just the scroll offset, *not* the "adjusted scroll offset". Scroll offset
-            // does not include the origin adjustment. That is instead baked already into offsetFromLayoutObject.
-            DoubleSize scrollOffset = scrollableArea->scrollOffset();
-            offset.expand(-scrollOffset.width(), -scrollOffset.height());
+            if (scrollableArea->usesCompositedScrolling()) {
+                // Note: this is just the scroll offset, *not* the "adjusted scroll offset". Scroll offset
+                // does not include the origin adjustment. That is instead baked already into offsetFromLayoutObject.
+                DoubleSize scrollOffset = scrollableArea->scrollOffset();
+                offset.expand(-scrollOffset.width(), -scrollOffset.height());
+            }
        }
    }
 }