Fix case where accessible scrollToMakeVisible didn't work.

Source/core/accessibility/AXObject.cpp

Index: Source/core/accessibility/AXObject.cpp
diff --git a/Source/core/accessibility/AXObject.cpp b/Source/core/accessibility/AXObject.cpp
index c296ddfbda8c8dcc53211a281e49abb716bcb372..b916975bb761f0f71cde29c3d51a83a8106ab80e 100644
--- a/Source/core/accessibility/AXObject.cpp
+++ b/Source/core/accessibility/AXObject.cpp
@@ -681,7 +681,7 @@ static int computeBestScrollOffset(int currentScrollOffset, int subfocusMin, int
 {
     int viewportSize = viewportMax - viewportMin;
 
-    // If the focus size is larger than the viewport size, shrink it in the
+    // If the object size is larger than the viewport size, shrink it in the

[aboxhall, 2014/10/17 19:30:09]

I think 'shrink it' is a little confusing - my reading is that it's not actually
changing the size of the object, but reducing the size of the portion of it to
be scrolled into view. Perhaps something like 'If the ..., consider only the
portion centred on the subfocus which will fit inside the viewport'?

[dmazzoni, 2014/10/17 19:41:35]

Done.

     // direction of subfocus.
     if (objectMax - objectMin > viewportSize) {
         // Subfocus must be within focus:
@@ -692,12 +692,12 @@ static int computeBestScrollOffset(int currentScrollOffset, int subfocusMin, int
         if (subfocusMax - subfocusMin > viewportSize)
             subfocusMax = subfocusMin + viewportSize;
 
-        if (subfocusMin + viewportSize > objectMax) {
-            objectMin = objectMax - viewportSize;
-        } else {
-            objectMin = subfocusMin;
-            objectMax = subfocusMin + viewportSize;
-        }
+        // Compute the size of an object centered on the subfocus, the size of the viewport.

[aboxhall, 2014/10/17 19:30:09]

Why do we want to centre it rather than keeping it at the nearest edge as we did
previously?

[dmazzoni, 2014/10/17 19:41:35]

I mentioned this in the change description - does that explanation make sense?

+        int centeredObjectMin = (subfocusMin + subfocusMax - viewportSize) / 2;
+        int centeredObjectMax = centeredObjectMin + viewportSize;
+
+        objectMin = std::max(objectMin, centeredObjectMin);
+        objectMax = std::min(objectMax, centeredObjectMax);
     }
 
     // Exit now if the focus is already within the viewport.
@@ -705,18 +705,8 @@ static int computeBestScrollOffset(int currentScrollOffset, int subfocusMin, int
         && objectMax - currentScrollOffset <= viewportMax)
         return currentScrollOffset;
 
-    // Scroll left if we're too far to the right.
-    if (objectMax - currentScrollOffset > viewportMax)
-        return objectMax - viewportMax;
-
-    // Scroll right if we're too far to the left.
-    if (objectMin - currentScrollOffset < viewportMin)
-        return objectMin - viewportMin;
-
-    ASSERT_NOT_REACHED();
-
-    // This shouldn't happen.
-    return currentScrollOffset;
+    // Center the object in the viewport.
+    return (objectMin + objectMax - viewportMin - viewportMax) / 2;
 }
 
 void AXObject::scrollToMakeVisibleWithSubFocus(const IntRect& subfocus) const
@@ -750,9 +740,16 @@ void AXObject::scrollToMakeVisibleWithSubFocus(const IntRect& subfocus) const
 
     scrollParent->scrollTo(IntPoint(desiredX, desiredY));
 
+    // Convert the subfocus into the coordinates of the scroll parent.
+    IntRect newSubfocus = subfocus;
+    IntRect newElementRect = pixelSnappedIntRect(elementRect());
+    IntRect scrollParentRect = pixelSnappedIntRect(scrollParent->elementRect());
+    newSubfocus.move(newElementRect.x(), newElementRect.y());
+    newSubfocus.move(-scrollParentRect.x(), -scrollParentRect.y());
+
     // Recursively make sure the scroll parent itself is visible.
     if (scrollParent->parentObject())
-        scrollParent->scrollToMakeVisible();
+        scrollParent->scrollToMakeVisibleWithSubFocus(newSubfocus);
 }
 
 void AXObject::scrollToGlobalPoint(const IntPoint& globalPoint) const