Bug 1367711 - Bail out early when initializing ReflowInput for a placeholder frame, which has a trivial Reflow method. r=mats

layout/generic/ReflowInput.cpp

@@ -391,6 +391,12 @@ ReflowInput::Init(nsPresContext*     aPresContext,
   mStyleText = mFrame->StyleText();
 
   LayoutFrameType type = mFrame->Type();
+  if (type == mozilla::LayoutFrameType::Placeholder) {
+    // Placeholders have a no-op Reflow method that doesn't need the rest of
+    // this initialization, so we bail out early.
+    ComputedBSize() = ComputedISize() = 0;
+    return;
+  }
 
   InitFrameType(type);
   InitCBReflowInput();

layout/generic/nsPlaceholderFrame.cpp

@@ -110,6 +110,10 @@ nsPlaceholderFrame::Reflow(nsPresContext*           aPresContext,
                            const ReflowInput& aReflowInput,
                            nsReflowStatus&          aStatus)
 {
+  // NOTE that the ReflowInput passed to this method is not fully initialized,
+  // on the grounds that reflowing a placeholder is a rather trivial operation.
+  // (See bug 1367711.)
+
 #ifdef DEBUG
   // We should be getting reflowed before our out-of-flow.
   // If this is our first reflow, and our out-of-flow has already received its