More fixes for position:fixed with root layer scrolling.

Source/core/layout/LayoutBox.cpp

Index: Source/core/layout/LayoutBox.cpp
diff --git a/Source/core/layout/LayoutBox.cpp b/Source/core/layout/LayoutBox.cpp
index 20aca77f26ceaab7599f6bade59be3bdd749f70e..60c7f8a8e333db021d469cf5ed08e0634bab99fe 100644
--- a/Source/core/layout/LayoutBox.cpp
+++ b/Source/core/layout/LayoutBox.cpp
@@ -2709,8 +2709,9 @@ LayoutUnit LayoutBox::containingBlockLogicalWidthForPositioned(const LayoutBoxMo
     if (style()->position() == FixedPosition && containingBlock->isLayoutView()) {
         const LayoutView* view = toLayoutView(containingBlock);
         if (FrameView* frameView = view->frameView()) {
-            LayoutRect viewportRect = LayoutRect(frameView->visibleContentRect());
-            return containingBlock->isHorizontalWritingMode() ? viewportRect.width() : viewportRect.height();
+            // Don't use visibleContentRect since the DeprecatedPaintLayer's size has not been set yet.
+            IntSize viewportSize = frameView->layoutViewportScrollableArea()->excludeScrollbars(frameView->frameRect().size());
+            return containingBlock->isHorizontalWritingMode() ? viewportSize.width() : viewportSize.height();
         }
     }
 
@@ -2752,8 +2753,9 @@ LayoutUnit LayoutBox::containingBlockLogicalHeightForPositioned(const LayoutBoxM
     if (style()->position() == FixedPosition && containingBlock->isLayoutView()) {
         const LayoutView* view = toLayoutView(containingBlock);
         if (FrameView* frameView = view->frameView()) {
-            LayoutRect viewportRect = LayoutRect(frameView->visibleContentRect());
-            return containingBlock->isHorizontalWritingMode() ? viewportRect.height() : viewportRect.width();
+            // Don't use visibleContentRect since the DeprecatedPaintLayer's size has not been set yet.
+            IntSize viewportSize = frameView->layoutViewportScrollableArea()->excludeScrollbars(frameView->frameRect().size());
+            return containingBlock->isHorizontalWritingMode() ? viewportSize.height() : viewportSize.width();
         }
     }