#612: M969874 M1525628

layout/generic/nsFrame.cpp

@@ -5889,6 +5889,11 @@ nsIFrame::IsBlockWrapper() const
           pseudoType == nsCSSAnonBoxes::cellContent);
 }
 
+bool nsIFrame::IsBlockFrameOrSubclass() {
+  nsBlockFrame* thisAsBlock = do_QueryFrame(this);
+  return !!thisAsBlock;
+}
+
 static nsIFrame*
 GetNearestBlockContainer(nsIFrame* frame)
 {

layout/generic/nsGfxScrollFrame.cpp

@@ -972,6 +972,29 @@ nsHTMLScrollFrame::AccessibleType()
 }
 #endif
 
+nscoord nsHTMLScrollFrame::GetLogicalBaseline(WritingMode aWritingMode) const {
+  // This function implements some of the spec text here:
+  //  https://drafts.csswg.org/css-align/#baseline-export
+  //
+  // Specifically: if our scrolled frame is a block, we just use the inherited
+  // GetLogicalBaseline() impl, which synthesizes a baseline from the
+  // margin-box. Otherwise, we defer to our scrolled frame, considering it
+  // to be scrolled to its initial scroll position.
+  if (mHelper.mScrolledFrame->IsBlockFrameOrSubclass()) {
+    return nsContainerFrame::GetLogicalBaseline(aWritingMode);
+  }
+
+  // OK, here's where we defer to our scrolled frame. We have to add our
+  // border BStart thickness to whatever it returns, to produce an offset in
+  // our frame-rect's coordinate system. (We don't have to add padding,
+  // because the scrolled frame handles our padding.)
+  LogicalMargin border = GetLogicalUsedBorder(aWritingMode);
+
+  return border.BStart(aWritingMode) +
+         mHelper.mScrolledFrame->GetLogicalBaseline(aWritingMode);
+}
+
+
 NS_QUERYFRAME_HEAD(nsHTMLScrollFrame)
   NS_QUERYFRAME_ENTRY(nsIAnonymousContentCreator)
   NS_QUERYFRAME_ENTRY(nsIScrollableFrame)

layout/generic/nsGfxScrollFrame.h

@@ -943,6 +943,8 @@ public:
   virtual mozilla::a11y::AccType AccessibleType() override;
 #endif
 
+  nscoord GetLogicalBaseline(mozilla::WritingMode aWritingMode) const override;
+
 protected:
   nsHTMLScrollFrame(nsStyleContext* aContext, bool aIsRoot);
   void SetSuppressScrollbarUpdate(bool aSuppress) {

layout/generic/nsIFrame.h

@@ -2162,6 +2162,18 @@ public:
    */
   bool IsBlockWrapper() const;
 
+  /**
+   * Returns true if the frame is an instance of nsBlockFrame or one of its
+   * subclasses.
+   *
+   * XXXdholbert this is non-const because it uses nsIFrame::QueryFrame which
+   * is non-const.  If we need this accessor to be 'const' down the road, the
+   * right way to do it would be to make the QueryFrame machinery
+   * const-friendly. But it may not be worth the trouble, because we rarely
+   * handle const frame pointers anyway.
+   */
+  bool IsBlockFrameOrSubclass();
+
   /**
    * Get this frame's CSS containing block.
    *