Set overflow: auto in StyleResolver::styleForDocument.

third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp

Index: third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
diff --git a/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp b/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
index 9d38a76f4e3c47728d7d4510f83dc997ba90f958..663da08ffdbcac0c18e408cbde4752e30d0589fe 100644
--- a/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
+++ b/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
@@ -622,6 +622,12 @@ PassRefPtr<ComputedStyle> StyleResolver::styleForDocument(Document& document) {
   documentStyle->setDisplay(EDisplay::Block);
   documentStyle->setPosition(AbsolutePosition);
 
+  // Document::inheritHtmlAndBodyElementStyles will set the final overflow
+  // style values, but they should initially be auto to avoid premature
+  // scrollbar removal in PaintLayerScrollableArea::updateAfterStyleChange.
+  documentStyle->setOverflowX(EOverflow::Auto);
+  documentStyle->setOverflowY(EOverflow::Auto);
+
   document.setupFontBuilder(*documentStyle);
 
   return documentStyle.release();