With a debug build of a recent pull of mozilla-central, I get the following warning: WARNING: Subdocument container has no frame: file nsDocumentViewer.cpp, line 2340 The stack trace is: #0 NS_DebugBreak_P (aSeverity=0, aStr=0xb57404a4 "Subdocument container has no frame", aExpr=0x0, aFile=0xb573fc82 "nsDocumentViewer.cpp", aLine=2340) at nsDebugImpl.cpp:255 #1 0xb506b601 in DocumentViewerImpl::FindContainerView (this=0xc793480) at nsDocumentViewer.cpp:2340 #2 0xb506f0a0 in DocumentViewerImpl::InitInternal (this=0xc793480, aParentWidget=0x0, aState=0x0, aBounds=@0xbffc74dc, aDoCreation=1, aInPrintPreview=0, aNeedMakeCX=1) at nsDocumentViewer.cpp:846 #3 0xb506f982 in DocumentViewerImpl::Init (this=0xc793480, aParentWidget=0x0, aBounds=@0xbffc74dc) at nsDocumentViewer.cpp:695 #4 0xb5ebd954 in nsDocShell::SetupNewViewer (this=0xb1bce78, aNewViewer=0xc793480) at nsDocShell.cpp:7280 #5 0xb5ec8062 in nsDocShell::Embed (this=0xb1bce78, aContentViewer=0xc793480, aCommand=0xb5f2a62a "", aExtraInfo=0x0) at nsDocShell.cpp:5475 #6 0xb5ec4800 in nsDocShell::CreateContentViewer (this=0xb1bce78, aContentType=0xb84af18 "text/html", request=0xb3ed710, aContentHandler=0xa0b1778) at nsDocShell.cpp:7065 #7 0xb5ee7081 in nsDSURIContentListener::DoContent (this=0xbd02010, aContentType=0xb84af18 "text/html", aIsContentPreferred=0, request=0xb3ed710, aContentHandler=0xa0b1778, aAbortProcess=0xbffc76ec) at nsDSURIContentListener.cpp:138 #8 0xb5eee888 in nsDocumentOpenInfo::TryContentListener (this=0xa0b1768, aListener=0xbd02010, aChannel=0xb3ed710) at nsURILoader.cpp:736 #9 0xb5eef217 in nsDocumentOpenInfo::DispatchContent (this=0xa0b1768, request=0xb3ed710, aCtxt=0x0) at nsURILoader.cpp:434 #10 0xb5eefbe5 in nsDocumentOpenInfo::OnStartRequest (this=0xa0b1768, request=0xb3ed710, aCtxt=0x0) at nsURILoader.cpp:280 #11 0xb5d99f97 in nsHttpChannel::CallOnStartRequest (this=0xb3ed6e0) at nsHttpChannel.cpp:849 #12 0xb5d9c3ba in nsHttpChannel::OnStartRequest (this=0xb3ed6e0, request=0xc90dee0, ctxt=0x0) at nsHttpChannel.cpp:5162 #13 0xb5cf0120 in nsInputStreamPump::OnStateStart (this=0xc90dee0) at nsInputStreamPump.cpp:439 #14 0xb5cf0b1e in nsInputStreamPump::OnInputStreamReady (this=0xc90dee0, stream=0xbb0e4ac) at nsInputStreamPump.cpp:395 #15 0xb7aa8881 in nsInputStreamReadyEvent::Run (this=0xb417140) at nsStreamUtils.cpp:112 #16 0xb7aca6bd in nsThread::ProcessNextEvent (this=0x9eb98f8, mayWait=1, result=0xbffc7b90) at nsThread.cpp:527 #17 0xb7a73642 in NS_ProcessNextEvent_P (thread=0xbffc726c, mayWait=1) at nsThreadUtils.cpp:230 #18 0xb5e76bb8 in nsBaseAppShell::Run (this=0xa384040) at nsBaseAppShell.cpp:170 #19 0xb4c1d446 in nsAppStartup::Run (this=0xa47fd88) at nsAppStartup.cpp:182 #20 0xb8057c3f in XRE_main (argc=4, argv=0xbffc8164, aAppData=0x9e378d8) at nsAppRunner.cpp:3418 #21 0x08048c1a in main (argc=4, argv=0xbffc8164) at nsBrowserApp.cpp:156

Note: we're disabling this warning by default due to its verbosity. To enable it all you have to do is set the 'layout' log module to emit up to the Warning level, for example: > NSPR_LOG_MODULES='layout:2' ./mach run enables the warning on debug builds.