Bug 916751: Do not use nsIFrame::MovePositionBy from nsLineLayout. r=dholbert

Since bug 916751 is hard (for me) to test, I haven't confirmed for sure
that this fixes the bug.

However, it fixes the assertions that bug 911786 part 3 triggers in
layout/base/crashtests/317934-1.html through this codepath.

layout/generic/nsIFrame.h

@@ -870,6 +870,12 @@ public:
    * Move the frame, accounting for relative positioning. Use this when
    * adjusting the frame's position by a known amount, to properly update its
    * saved normal position (see GetNormalPosition below).
+   *
+   * This must be used only when moving a frame *after*
+   * nsHTMLReflowState::ApplyRelativePositioning is called.  When moving
+   * a frame during the reflow process prior to calling
+   * nsHTMLReflowState::ApplyRelativePositioning, the position should
+   * simply be adjusted directly (e.g., using SetPosition()).
    */
   void MovePositionBy(const nsPoint& aTranslation);
 

layout/generic/nsLineLayout.cpp

@@ -2196,7 +2196,13 @@ nsLineLayout::VerticalAlignFrames(PerSpanData* psd)
 
 static void SlideSpanFrameRect(nsIFrame* aFrame, nscoord aDeltaWidth)
 {
-  aFrame->MovePositionBy(nsPoint(-aDeltaWidth, 0));
+  // This should not use nsIFrame::MovePositionBy because it happens
+  // prior to relative positioning.  In particular, because
+  // nsBlockFrame::PlaceLine calls aLineLayout.TrimTrailingWhiteSpace()
+  // prior to calling aLineLayout.RelativePositionFrames().
+  nsPoint p = aFrame->GetPosition();
+  p.x -= aDeltaWidth;
+  aFrame->SetPosition(p);
 }
 
 bool