Bug 941356 - Fix start offset of transformed drag images. r=botond

I'm not sure whether we should deal with ancestor scales and such. There
seemed to be a discussion about that in D77436 but dealing with
partially-3d-transformed content sounds like a massive pain. For now
this fixes the start point, which is a progression.

Differential Revision: https://phabricator.services.mozilla.com/D106896

dom/base/DOMIntersectionObserver.cpp

@@ -636,10 +636,7 @@ void DOMIntersectionObserver::Update(Document* aDocument,
 
       // 2.3. Let targetRect be a DOMRectReadOnly obtained by running the
       // getBoundingClientRect() algorithm on target.
-      targetRect = nsLayoutUtils::GetAllInFlowRectsUnion(
-          targetFrame,
-          nsLayoutUtils::GetContainingBlockForClientRect(targetFrame),
-          nsLayoutUtils::RECTS_ACCOUNT_FOR_TRANSFORMS);
+      targetRect = targetFrame->GetBoundingClientRect();
 
       // 2.4. Let intersectionRect be the result of running the compute the
       // intersection algorithm on target.

dom/base/Element.cpp

@@ -1014,10 +1014,7 @@ already_AddRefed<DOMRect> Element::GetBoundingClientRect() {
     return rect.forget();
   }
 
-  nsRect r = nsLayoutUtils::GetAllInFlowRectsUnion(
-      frame, nsLayoutUtils::GetContainingBlockForClientRect(frame),
-      nsLayoutUtils::RECTS_ACCOUNT_FOR_TRANSFORMS);
-  rect->SetLayoutRect(r);
+  rect->SetLayoutRect(frame->GetBoundingClientRect());
   return rect.forget();
 }
 

layout/base/PresShell.cpp

@@ -4844,6 +4844,8 @@ UniquePtr<RangePaintInfo> PresShell::CreateRangePaintInfo(
   // appropriate nsDisplayAsyncZoom display items. This code handles the general
   // case with nested async zooms (even though that never actually happens),
   // because it fell out of the implementation for free.
+  //
+  // TODO: Do we need to do the same for ancestor transforms?
   for (nsPresContext* ctx = GetPresContext(); ctx;
        ctx = ctx->GetParentPresContext()) {
     PresShell* shell = ctx->PresShell();
@@ -4884,7 +4886,7 @@ UniquePtr<RangePaintInfo> PresShell::CreateRangePaintInfo(
   // determine the offset of the reference frame for the display list
   // to the root frame. This will allow the coordinates used when painting
   // to all be offset from the same point
-  info->mRootOffset = ancestorFrame->GetOffsetTo(rootFrame);
+  info->mRootOffset = ancestorFrame->GetBoundingClientRect().TopLeft();
   rangeRect.MoveBy(info->mRootOffset);
   aSurfaceRect.UnionRect(aSurfaceRect, rangeRect);
 

layout/generic/nsIFrame.cpp

@@ -7403,6 +7403,12 @@ nsRect nsIFrame::GetNormalRect() const {
   return GetRect();
 }
 
+nsRect nsIFrame::GetBoundingClientRect() {
+  return nsLayoutUtils::GetAllInFlowRectsUnion(
+      this, nsLayoutUtils::GetContainingBlockForClientRect(this),
+      nsLayoutUtils::RECTS_ACCOUNT_FOR_TRANSFORMS);
+}
+
 nsPoint nsIFrame::GetPositionIgnoringScrolling() const {
   return GetParent() ? GetParent()->GetPositionOfChildIgnoringScrolling(this)
                      : GetPosition();

layout/generic/nsIFrame.h

@@ -1284,6 +1284,11 @@ class nsIFrame : public nsQueryFrame {
    */
   nsRect GetNormalRect() const;
 
+  /**
+   * Returns frame's rect as required by the GetBoundingClientRect() DOM API.
+   */
+  nsRect GetBoundingClientRect();
+
   /**
    * Return frame's position without relative positioning.
    * If aHasProperty is provided, returns whether the normal position

widget/nsBaseDragService.cpp

@@ -724,9 +724,8 @@ nsresult nsBaseDragService::DrawDrag(nsINode* aDOMNode,
       // otherwise, there was no region so just set the rectangle to
       // the size of the primary frame of the content.
       nsCOMPtr<nsIContent> content = do_QueryInterface(dragNode);
-      nsIFrame* frame = content->GetPrimaryFrame();
-      if (frame) {
-        presLayoutRect = frame->GetRect();
+      if (nsIFrame* frame = content->GetPrimaryFrame()) {
+        presLayoutRect = frame->GetBoundingClientRect();
       }
     }