Bug 41708, support scrolling when hovering over the edge of a scrollframe while dragging, r=smaug

dom/events/EventStateManager.cpp

@@ -3331,6 +3331,20 @@ EventStateManager::PostHandleEvent(nsPresContext* aPresContext,
     {
       NS_ASSERTION(aEvent->mClass == eDragEventClass, "Expected a drag event");
 
+      // Check if the drag is occurring inside a scrollable area. If so, scroll
+      // the area when the mouse is near the edges.
+      if (mCurrentTarget && aEvent->mMessage == eDragOver) {
+        nsIFrame* checkFrame = mCurrentTarget;
+        while (checkFrame) {
+          nsIScrollableFrame* scrollFrame = do_QueryFrame(checkFrame);
+          // Break out so only the innermost scrollframe is scrolled.
+          if (scrollFrame && scrollFrame->DragScroll(aEvent)) {
+            break;
+          }
+          checkFrame = checkFrame->GetParent();
+        }
+      }
+
       nsCOMPtr<nsIDragSession> dragSession = nsContentUtils::GetDragSession();
       if (!dragSession)
         break;

layout/generic/nsGfxScrollFrame.cpp

@@ -6159,3 +6159,60 @@ ScrollFrameHelper::UsesContainerScrolling() const
   }
   return false;
 }
+
+bool
+ScrollFrameHelper::DragScroll(WidgetEvent* aEvent)
+{
+  // Dragging is allowed while within a 20 pixel border. Note that device pixels
+  // are used so that the same margin is used even when zoomed in or out.
+  nscoord margin = 20 * mOuter->PresContext()->AppUnitsPerDevPixel();
+
+  // Don't drag scroll for small scrollareas.
+  if (mScrollPort.width < margin * 2 || mScrollPort.height < margin * 2) {
+    return false;
+  }
+
+  // If willScroll is computed as false, then the frame is already scrolled as
+  // far as it can go in both directions. Return false so that an ancestor
+  // scrollframe can scroll instead.
+  bool willScroll = false;
+  nsPoint pnt = nsLayoutUtils::GetEventCoordinatesRelativeTo(aEvent, mOuter);
+  nsPoint scrollPoint = GetScrollPosition();
+  nsRect rangeRect = GetScrollRangeForClamping();
+
+  // Only drag scroll when a scrollbar is present.
+  nsPoint offset;
+  if (mHasHorizontalScrollbar) {
+    if (pnt.x >= mScrollPort.x && pnt.x <= mScrollPort.x + margin) {
+      offset.x = -margin;
+      if (scrollPoint.x > 0) {
+        willScroll = true;
+      }
+    } else if (pnt.x >= mScrollPort.XMost() - margin && pnt.x <= mScrollPort.XMost()) {
+      offset.x = margin;
+      if (scrollPoint.x < rangeRect.width) {
+        willScroll = true;
+      }
+    }
+  }
+
+  if (mHasVerticalScrollbar) {
+    if (pnt.y >= mScrollPort.y && pnt.y <= mScrollPort.y + margin) {
+      offset.y = -margin;
+      if (scrollPoint.y > 0) {
+        willScroll = true;
+      }
+    } else if (pnt.y >= mScrollPort.YMost() - margin && pnt.y <= mScrollPort.YMost()) {
+      offset.y = margin;
+      if (scrollPoint.y < rangeRect.height) {
+        willScroll = true;
+      }
+    }
+  }
+
+  if (offset.x || offset.y) {
+    ScrollTo(GetScrollPosition() + offset, nsIScrollableFrame::NORMAL);
+  }
+
+  return willScroll;
+}

layout/generic/nsGfxScrollFrame.h

@@ -460,6 +460,8 @@ public:
     return mSuppressScrollbarRepaints;
   }
 
+  bool DragScroll(WidgetEvent* aEvent);
+
   // owning references to the nsIAnonymousContentCreator-built content
   nsCOMPtr<nsIContent> mHScrollbarContent;
   nsCOMPtr<nsIContent> mVScrollbarContent;
@@ -1033,6 +1035,10 @@ public:
     return mHelper.GetScrollSnapInfo();
   }
 
+  virtual bool DragScroll(mozilla::WidgetEvent* aEvent) override {
+    return mHelper.DragScroll(aEvent);
+  }
+
 #ifdef DEBUG_FRAME_DUMP
   virtual nsresult GetFrameName(nsAString& aResult) const override;
 #endif
@@ -1456,6 +1462,10 @@ public:
     return mHelper.GetScrollSnapInfo();
   }
 
+  virtual bool DragScroll(mozilla::WidgetEvent* aEvent) override {
+    return mHelper.DragScroll(aEvent);
+  }
+
 #ifdef DEBUG_FRAME_DUMP
   virtual nsresult GetFrameName(nsAString& aResult) const override;
 #endif

layout/generic/nsIScrollableFrame.h

@@ -475,6 +475,14 @@ public:
   virtual ScrollSnapInfo GetScrollSnapInfo() const = 0;
 
   virtual void SetScrollsClipOnUnscrolledOutOfFlow() = 0;
+
+  /**
+   * Given the drag event aEvent, determine whether the mouse is near the edge
+   * of the scrollable area, and scroll the view in the direction of that edge
+   * if so. If scrolling occurred, true is returned. When false is returned, the
+   * caller should look for an ancestor to scroll.
+   */
+  virtual bool DragScroll(mozilla::WidgetEvent* aEvent) = 0;
 };
 
 #endif