https://chromium-review.googlesource.com/c/chromium/src/+/922662

I'm not able to repro this. What's the full stack of the hanging threads in question?

Stack:
   base.dll!base::PlatformThread::Join(base::PlatformThreadHandle thread_handle)
   base.dll!base::Thread::Stop()
   base.dll!base::Thread::~Thread()
   base.dll!base::Thread::~Thread()
   shell_dialogs.dll!ui::BaseShellDialogImpl::EndRun(ui::BaseShellDialogImpl::RunState run_state)
   shell_dialogs.dll!`anonymous namespace'::SelectFileDialogImpl::FileNotSelected(void * params, ui::BaseShellDialogImpl::RunState run_state)
   shell_dialogs.dll!base::internal::FunctorTraits<void ((anonymous namespace)::SelectFileDialogImpl::*)(void *, ui::BaseShellDialogImpl::RunState) __attribute__((thiscall)),void>::Invoke<const scoped_refptr<(anonymous namespace)::SelectFileDialogImpl> &,void *const &,const ui::BaseShellDialogImpl::RunState &>(void(`anonymous namespace'::SelectFileDialogImpl::*)(void *, ui::BaseShellDialogImpl::RunState) method, const scoped_refptr<(anonymous namespace)::SelectFileDialogImpl> & receiver_ptr, void * const & args, const ui::BaseShellDialogImpl::RunState & args)
   shell_dialogs.dll!base::internal::InvokeHelper<0,void>::MakeItSo<void ((anonymous namespace)::SelectFileDialogImpl::*const &)(void *, ui::BaseShellDialogImpl::RunState) __attribute__((thiscall)),const scoped_refptr<(anonymous namespace)::SelectFileDialogImpl> &,void *const &,const ui::BaseShellDialogImpl::RunState &>(void(`anonymous namespace'::SelectFileDialogImpl::*)(void *, ui::BaseShellDialogImpl::RunState) & functor, const scoped_refptr<(anonymous namespace)::SelectFileDialogImpl> & args, void * const & args, const ui::BaseShellDialogImpl::RunState & args) 
   shell_dialogs.dll!base::internal::Invoker<base::internal::BindState<void ((anonymous namespace)::SelectFileDialogImpl::*)(void *, ui::BaseShellDialogImpl::RunState) __attribute__((thiscall)),scoped_refptr<(anonymous namespace)::SelectFileDialogImpl>,void *,ui::BaseShellDialogImpl::RunState>,void ()>::RunImpl<void ((anonymous namespace)::SelectFileDialogImpl::*const &)(void *, ui::BaseShellDialogImpl::RunState) __attribute__((thiscall)),const std::tuple<scoped_refptr<(anonymous namespace)::SelectFileDialogImpl>,void *,ui::BaseShellDialogImpl::RunState> &,0,1,2>(void(`anonymous namespace'::SelectFileDialogImpl::*)(void *, ui::BaseShellDialogImpl::RunState) & functor, const std::tuple<scoped_refptr<(anonymous namespace)::SelectFileDialogImpl>,void *,ui::BaseShellDialogImpl::RunState> & bound, std::integer_sequence<unsigned int,0,1,2>) 
   shell_dialogs.dll!base::internal::Invoker<base::internal::BindState<void ((anonymous namespace)::SelectFileDialogImpl::*)(void *, ui::BaseShellDialogImpl::RunState) __attribute__((thiscall)),scoped_refptr<(anonymous namespace)::SelectFileDialogImpl>,void *,ui::BaseShellDialogImpl::RunState>,void ()>::Run(base::internal::BindStateBase * base) 
   base.dll!base::OnceCallback<void ()>::Run() 
   base.dll!base::debug::TaskAnnotator::RunTask(const char * queue_function, base::PendingTask * pending_task) 
   base.dll!base::internal::IncomingTaskQueue::RunTask(base::PendingTask * pending_task) 
   base.dll!base::MessageLoop::RunTask(base::PendingTask * pending_task) 
   base.dll!base::MessageLoop::DeferOrRunPendingTask(base::PendingTask pending_task) 
   base.dll!base::MessageLoop::DoWork()
   base.dll!base::MessagePumpForUI::DoRunLoop()
   base.dll!base::MessagePumpWin::Run(base::MessagePump::Delegate * delegate)
   base.dll!base::MessageLoop::Run(bool application_tasks_allowed) 
   base.dll!base::RunLoop::Run() 
   browser.dll!ChromeBrowserMainParts::MainMessageLoopRun(int * result_code) 
   content.dll!content::BrowserMainLoop::RunMainMessageLoopParts() 
   content.dll!content::BrowserMainRunnerImpl::Run() 
   content.dll!content::BrowserMain(const content::MainFunctionParams & parameters)
   content.dll!content::RunNamedProcessTypeMain(const std::basic_string<char,std::char_traits<char>,std::allocator<char> > & process_type, const content::MainFunctionParams & main_function_params, content::ContentMainDelegate * delegate)
   content.dll!content::ContentMainRunnerImpl::Run() 
   content.dll!content::ContentServiceManagerMainDelegate::RunEmbedderProcess()
   embedder.dll!service_manager::Main(const service_manager::MainParams & params)
   content.dll!content::ContentMain(const content::ContentMainParams & params) 
   browser.dll!ChromeMain(HINSTANCE__ * instance, sandbox::SandboxInterfaceInfo * sandbox_info, __int64 exe_entry_point_ticks) 
   browser.exe!MainDllLoader::Launch(HINSTANCE__ * instance, base::TimeTicks exe_entry_point_ticks)
   browser.exe!wWinMain(HINSTANCE__ * instance, HINSTANCE__ * prev, wchar_t *, int)

Thank you for providing more feedback. Adding requester "rbasuvula@chromium.org" to the cc list and removing "Needs-Feedback" label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Also, reproduced on Win7.
Tips & tricks:
  1) Here we have some kind of a race, so try to drag & drop several large files before closing the dialog.
  2) Close dialog (quickly) right after files were dropped.
  3) Sometimes it takes several attempts to reproduce the issue.

Unable to reproduce the issue on Windows 10 with Chrome Beta #65.0.3325.73 Dev #65.0.3325.73 Canary #66.0.3350.0.

svartmetal@ Could you please help us with the crash ID from chrome://crashes for further triage.

#4. What's the stack of the thread being joined? That will identify the deadlock.

 Issue 645913  gives us a good repro for this one.