Implement Document.scrollingElement API behind experimental feature flag

Source/core/dom/Element.cpp

Index: Source/core/dom/Element.cpp
diff --git a/Source/core/dom/Element.cpp b/Source/core/dom/Element.cpp
index 71191c2d41e13f567b68072831047999af2ca051..5df6909caedb43f7aa41d396ad4d3224dec2cf73 100644
--- a/Source/core/dom/Element.cpp
+++ b/Source/core/dom/Element.cpp
@@ -711,20 +711,15 @@ double Element::scrollLeft()
 {
     document().updateLayoutIgnorePendingStylesheets();
 
-    if (document().documentElement() != this) {
-        if (LayoutBox* box = layoutBox())
-            return adjustScrollForAbsoluteZoom(box->scrollLeft(), *box);
-        return 0;
-    }
-
-    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
-        if (document().inQuirksMode())
-            return 0;
-
+    if (document().scrollingElement() == this) {
         if (LocalDOMWindow* window = document().domWindow())
             return window->scrollX();

[tdresser, 2015/04/13 13:45:30]

I think these would be somewhat easier to read as:
if (document().domWindow())
  return document().domWindow()->scrollX();

Using assignment in a conditional makes it a little harder to read, and I don't
think the benefit is worth it.

[Rick Byers, 2015/04/13 20:08:31]

Done for these cases 'window' is used only once.  For the cases like 'box' below
and setScrollTop/Left uses of 'window' we need to use the local twice and it
seems redundant to repeat the same expression three times in two lines.  WDYT?

I've had reviewers ask me to use this style before, but I can't find anything in
any of the style guides so perhaps it's just reviewer preference...

+        return 0;
     }
 
+    if (LayoutBox* box = layoutBox())
+        return adjustScrollForAbsoluteZoom(box->scrollLeft(), *box);
+
     return 0;
 }
 
@@ -732,20 +727,15 @@ double Element::scrollTop()
 {
     document().updateLayoutIgnorePendingStylesheets();
 
-    if (document().documentElement() != this) {
-        if (LayoutBox* box = layoutBox())
-            return adjustScrollForAbsoluteZoom(box->scrollTop(), *box);
-        return 0;
-    }
-
-    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
-        if (document().inQuirksMode())
-            return 0;
-
+    if (document().scrollingElement() == this) {
         if (LocalDOMWindow* window = document().domWindow())
             return window->scrollY();
+        return 0;
     }
 
+    if (LayoutBox* box = layoutBox())
+        return adjustScrollForAbsoluteZoom(box->scrollTop(), *box);
+
     return 0;
 }
 
@@ -756,19 +746,13 @@ void Element::setScrollLeft(double newLeft)
     if (std::isnan(newLeft))
         return;
 
-    if (document().documentElement() != this) {
+    if (document().scrollingElement() == this) {
+        if (LocalDOMWindow* window = document().domWindow())
+            window->scrollTo(newLeft, window->scrollY());
+    } else {
         LayoutBox* box = layoutBox();
         if (box)
             box->setScrollLeft(LayoutUnit::fromFloatRound(newLeft * box->style()->effectiveZoom()));
-        return;
-    }
-
-    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
-        if (document().inQuirksMode())
-            return;
-
-        if (LocalDOMWindow* window = document().domWindow())
-            window->scrollTo(newLeft, window->scrollY());
     }
 }
 
@@ -779,25 +763,26 @@ void Element::setScrollTop(double newTop)
     if (std::isnan(newTop))
         return;
 
-    if (document().documentElement() != this) {
+    if (document().scrollingElement() == this) {
+        if (LocalDOMWindow* window = document().domWindow())
+            window->scrollTo(window->scrollX(), newTop);
+    } else {
         LayoutBox* box = layoutBox();
         if (box)
             box->setScrollTop(LayoutUnit::fromFloatRound(newTop * box->style()->effectiveZoom()));
-        return;
-    }
-
-    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
-        if (document().inQuirksMode())
-            return;
-
-        if (LocalDOMWindow* window = document().domWindow())
-            window->scrollTo(window->scrollX(), newTop);
     }
 }
 
 int Element::scrollWidth()
 {
     document().updateLayoutIgnorePendingStylesheets();
+
+    if (document().scrollingElement() == this) {
+        if (FrameView* view = document().view())
+            return adjustForAbsoluteZoom(view->contentsWidth(), document().frame()->pageZoomFactor());
+        return 0;
+    }
+
     if (LayoutBox* box = layoutBox())
         return adjustLayoutUnitForAbsoluteZoom(box->scrollWidth(), *box).round();
     return 0;
@@ -806,6 +791,13 @@ int Element::scrollWidth()
 int Element::scrollHeight()
 {
     document().updateLayoutIgnorePendingStylesheets();
+
+    if (document().scrollingElement() == this) {
+        if (FrameView* view = document().view())
+            return adjustForAbsoluteZoom(view->contentsHeight(), document().frame()->pageZoomFactor());
+        return 0;
+    }
+
     if (LayoutBox* box = layoutBox())
         return adjustLayoutUnitForAbsoluteZoom(box->scrollHeight(), *box).round();
     return 0;
@@ -825,15 +817,10 @@ void Element::scrollBy(const ScrollToOptions& scrollToOptions)
     // the compositing update. See http://crbug.com/420741.
     document().updateLayoutIgnorePendingStylesheets();
 
-    if (document().documentElement() != this) {
-        scrollLayoutBoxBy(scrollToOptions);
-        return;
-    }
-
-    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
-        if (document().inQuirksMode())
-            return;
+    if (document().scrollingElement() == this) {
         scrollFrameBy(scrollToOptions);
+    } else {
+        scrollLayoutBoxBy(scrollToOptions);
     }
 }
 
@@ -851,15 +838,10 @@ void Element::scrollTo(const ScrollToOptions& scrollToOptions)
     // the compositing update. See http://crbug.com/420741.
     document().updateLayoutIgnorePendingStylesheets();
 
-    if (document().documentElement() != this) {
-        scrollLayoutBoxTo(scrollToOptions);
-        return;
-    }
-
-    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
-        if (document().inQuirksMode())
-            return;
+    if (document().scrollingElement() == this) {
         scrollFrameTo(scrollToOptions);
+    } else {
+        scrollLayoutBoxTo(scrollToOptions);
     }
 }