diff --git a/layout/base/nsPresShell.cpp b/layout/base/nsPresShell.cpp
--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -6899,6 +6899,8 @@
 PRBool
 PresShell::DoReflow(nsIFrame* target, PRBool aInterruptible)
 {
+  fprintf(stderr, "start PresShell::DoReflow (%d)\n", aInterruptible);
+
   nsIFrame* rootFrame = FrameManager()->GetRootFrame();
 
   nsCOMPtr<nsIRenderingContext> rcx;
@@ -7022,6 +7024,8 @@
   }
 
   mPresContext->SetInterruptState(PR_FALSE);
+  fprintf(stderr, "end   PresShell::DoReflow (%d)\n\n", interrupted);
+
   return !interrupted;
 }