--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -10973,31 +10973,34 @@ nsIPresShell::FontSizeInflationEnabled()
 
 void
 nsIPresShell::HandleGlobalZoom()
 {
   if (nsLayoutUtils::BrowserGlobalZoom() == 0) {
     return;
   }
 
+  PRES_LOG("%p: applying GlobalZoom\n", this);
   float zoomFactor = nsLayoutUtils::BrowserGlobalZoom() / 100.0f;
 
   MOZ_ASSERT(mDocument, "our document should not be null");
+  PRES_LOG("%p: is synthetic document: %s\n", this, mDocument->IsSyntheticDocument()?"true":"false");
 
   if (!mDocIsFontSizeInflationEligible && !(mDocument->IsSyntheticDocument())) {
     SetGlobalZoom(zoomFactor);
   } else {
     SetGlobalZoom(1.0f);
   }
 }
 
 void
 nsIPresShell::SetGlobalZoom(const float& aZoomFactor)
 {
   MOZ_ASSERT(mPresContext, "our pres context should not be null");
+  PRES_LOG("%p: SetGlobalZoom: %f\n", this, aZoomFactor);
 
   if (nsLayoutUtils::BrowserGlobalZoomFull()) {
     mPresContext->SetFullZoom(aZoomFactor);
     mPresContext->SetTextZoom(1.0f);
   } else {
     mPresContext->SetFullZoom(1.0f);
     mPresContext->SetTextZoom(aZoomFactor);
   }