lgrey@ - can you take a look?

Marking as stable blocker so it's on our radar, in case there's a regression with 56. 

Here's the relevant portion of the UI thread's stack trace:

    7752 Thread_25616   DispatchQueue_1: com.apple.main-thread  (serial)
    + 7752 start  (in libdyld.dylib) + 1  [0x7fffb42dd255]
    +   7752 main  (in Google Chrome) + 522  [chrome_exe_main_mac.c:89]
    +     7752 ChromeMain  (in Google Chrome Framework) + 60  [chrome_main.cc:108]
    +       7752 content::ContentMain(content::ContentMainParams const&)  (in Google Chrome Framework)  load address 0x1060d9000 + 0x13dc716  [content_main.cc:20]
    +         7752 content::ContentMainRunnerImpl::Run()  (in Google Chrome Framework)  load address 0x1060d9000 + 0x13dd47d  [content_main_runner.cc:774]
    +           7752 content::BrowserMain(content::MainFunctionParams const&)  (in Google Chrome Framework)  load address 0x1060d9000 + 0xc7bbec  [browser_main.cc:46]
    +             7752 content::BrowserMainRunnerImpl::Run()  (in Google Chrome Framework)  load address 0x1060d9000 + 0xc821e2  [memory:2761]
    +               7752 content::BrowserMainLoop::RunMainMessageLoopParts()  (in Google Chrome Framework)  load address 0x1060d9000 + 0xc7fab4  [browser_main_loop.cc:986]
    +                 7752 ChromeBrowserMainParts::MainMessageLoopRun(int*)  (in Google Chrome Framework)  load address 0x1060d9000 + 0x1425555  [chrome_browser_main.cc:2014]
    +                   7752 base::RunLoop::Run()  (in Google Chrome Framework)  load address 0x1060d9000 + 0x1870983  [run_loop.cc:36]
    +                     7752 base::MessagePumpCFRunLoopBase::Run(base::MessagePump::Delegate*)  (in Google Chrome Framework)  load address 0x1060d9000 + 0x185538c  [message_pump_mac.mm:218]
    +                       7752 base::MessagePumpNSApplication::DoRun(base::MessagePump::Delegate*)  (in Google Chrome Framework)  load address 0x1060d9000 + 0x1855cee  [message_pump_mac.mm:654]
    +                         7752 -[NSApplication run]  (in AppKit) + 1002  [0x7fff9c8054b1]
    +                           7752 -[BrowserCrApplication sendEvent:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x1420368  [chrome_browser_application_mac.mm:374]
    +                             7752 base::mac::CallWithEHFrame(void () block_pointer)  (in Google Chrome Framework)  load address 0x1060d9000 + 0x1847b0a  []
    +                               7752 __34-[BrowserCrApplication sendEvent:]_block_invoke  (in Google Chrome Framework)  load address 0x1060d9000 + 0x142041c  [chrome_browser_application_mac.mm:370]
    +                                 7752 -[NSApplication(NSEvent) sendEvent:]  (in AppKit) + 1145  [0x7fff9cf890a9]
    +                                   7752 -[ChromeEventProcessingWindow sendEvent:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x3d90cff  [chrome_event_processing_window.mm:73]
    +                                     7752 -[NSWindow(NSEventRouting) sendEvent:]  (in AppKit) + 541  [0x7fff9d100772]
    +                                       7752 -[NSWindow(NSEventRouting) _reallySendEvent:isDelayedEvent:]  (in AppKit) + 1942  [0x7fff9d1012d4]
    +                                         7752 -[NSWindow(NSEventRouting) _handleMouseDownEvent:isDelayedEvent:]  (in AppKit) + 6341  [0x7fff9d104ab7]
    +                                           7752 -[DraggableButton mouseDown:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x3dacbe1  [draggable_button.mm:44]
    +                                             7752 -[DraggableButtonImpl mouseDownImpl:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x3dacf91  [draggable_button_mixin.mm:97]
    +                                               7752 -[DraggableButtonImpl performMouseDownAction:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x3dad450  [draggable_button_mixin.mm:244]
    +                                                 5277 -[BookmarkButton performMouseDownAction:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x3d7410d  [bookmark_button.mm:291]
    +                                                 ! 4776 objc_msgSend  (in libobjc.A.dylib) + 46,0,...  [0x7fffb39edb6e,0x7fffb39edb40,...]
    +                                                 ! 501 -[NSView window]  (in AppKit) + 4,0,...  [0x7fff9c7ebdc5,0x7fff9c7ebdc1,...]
    +                                                 1470 -[BookmarkButton performMouseDownAction:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x3d7411c  [bookmark_button.mm:291]
    +                                                 ! 1470 objc_msgSend  (in libobjc.A.dylib) + 114,0,...  [0x7fffb39edbb2,0x7fffb39edb40,...]
    +                                                 518 -[BookmarkButton performMouseDownAction:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x3d74100  [bookmark_button.mm:321]
    +                                                 483 -[BookmarkButton performMouseDownAction:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x3d7410d  [bookmark_button.mm:291]
    +                                                 2 -[BookmarkButton performMouseDownAction:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x3d7411c  [bookmark_button.mm:291]
    +                                                 1 -[BookmarkButton performMouseDownAction:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x3d7410a  [bookmark_button.mm:291]
    +                                                 1 -[BookmarkButton performMouseDownAction:]  (in Google Chrome Framework)  load address 0x1060d9000 + 0x3d74112  [bookmark_button.mm:291]

klobag@ have you been able to repro this? If you can, does clicking the mouse/trackpad on the beachballing window break you out?

Thanks for the investigation.

FYI: Your bug is labelled as Release Block Stable , we will be shipping M56 to stable soon. So it would be great to have a fix ASAP.

Grace, has this happened again? 

@Grace, Could you please respond as per comment #7

Since there is no relevant updates, removing the RBS. 
Please feel to add if needed.

Sorry, missed ping earlier. It hasn't happened again.

Let's close this then.