Bug 975962 - Allow panning/zooming around the initial viewport for overflow:hidden elements. r=botond,tn

layout/base/nsDisplayList.cpp

@@ -679,16 +679,18 @@ static void RecordFrameMetrics(nsIFrame* aForFrame,
 
   if (scrollableFrame) {
     nsRect contentBounds = scrollableFrame->GetScrollRange();
+    nsPoint scrollPosition = scrollableFrame->GetScrollPosition();
     if (scrollableFrame->GetScrollbarStyles().mVertical == NS_STYLE_OVERFLOW_HIDDEN) {
-      metrics.SetDisableScrollingY(true);
+      contentBounds.y = scrollPosition.y;
+      contentBounds.height = 0;
     }
     if (scrollableFrame->GetScrollbarStyles().mHorizontal == NS_STYLE_OVERFLOW_HIDDEN) {
-      metrics.SetDisableScrollingX(true);
+      contentBounds.x = scrollPosition.x;
+      contentBounds.width = 0;
     }
     contentBounds.width += scrollableFrame->GetScrollPortRect().width;
     contentBounds.height += scrollableFrame->GetScrollPortRect().height;
     metrics.mScrollableRect = CSSRect::FromAppUnits(contentBounds);
-    nsPoint scrollPosition = scrollableFrame->GetScrollPosition();
     metrics.mScrollOffset = CSSPoint::FromAppUnits(scrollPosition);
 
     // If the frame was scrolled since the last layers update, and by

widget/xpwidgets/APZCCallbackHelper.cpp

@@ -105,6 +105,23 @@ ScrollFrameTo(nsIScrollableFrame* aFrame, const CSSPoint& aPoint, bool& aSuccess
     return aPoint;
   }
 
+  CSSPoint targetScrollPosition = aPoint;
+
+  // If the frame is overflow:hidden on a particular axis, we don't want to allow
+  // user-driven scroll on that axis. Simply set the scroll position on that axis
+  // to whatever it already is. Note that this will leave the APZ's async scroll
+  // position out of sync with the gecko scroll position, but APZ can deal with that
+  // (by design). Note also that when we run into this case, even if both axes
+  // have overflow:hidden, we want to set aSuccessOut to true, so that the displayport
+  // follows the async scroll position rather than the gecko scroll position.
+  CSSPoint geckoScrollPosition = CSSPoint::FromAppUnits(aFrame->GetScrollPosition());
+  if (aFrame->GetScrollbarStyles().mVertical == NS_STYLE_OVERFLOW_HIDDEN) {
+    targetScrollPosition.y = geckoScrollPosition.y;
+  }
+  if (aFrame->GetScrollbarStyles().mHorizontal == NS_STYLE_OVERFLOW_HIDDEN) {
+    targetScrollPosition.x = geckoScrollPosition.x;
+  }
+
   // If the scrollable frame is currently in the middle of an async or smooth
   // scroll then we don't want to interrupt it (see bug 961280).
   // Also if the scrollable frame got a scroll request from something other than us
@@ -112,13 +129,14 @@ ScrollFrameTo(nsIScrollableFrame* aFrame, const CSSPoint& aPoint, bool& aSuccess
   // because we'll clobber that one, which is bad.
   if (!aFrame->IsProcessingAsyncScroll() &&
      (!aFrame->OriginOfLastScroll() || aFrame->OriginOfLastScroll() == nsGkAtoms::apz)) {
-    aFrame->ScrollToCSSPixelsApproximate(aPoint, nsGkAtoms::apz);
+    aFrame->ScrollToCSSPixelsApproximate(targetScrollPosition, nsGkAtoms::apz);
+    geckoScrollPosition = CSSPoint::FromAppUnits(aFrame->GetScrollPosition());
     aSuccessOut = true;
   }
   // Return the final scroll position after setting it so that anything that relies
   // on it can have an accurate value. Note that even if we set it above re-querying it
   // is a good idea because it may have gotten clamped or rounded.
-  return CSSPoint::FromAppUnits(aFrame->GetScrollPosition());
+  return geckoScrollPosition;
 }
 
 void