Bug in libX11 or something near ?

Jon Turney jon.turney@dronecode.org.uk
Sun Jun 23 17:57:00 GMT 2019

On 18/06/2019 17:07, Philippe wrote:
> I'm using Cygwin/X and identified a critical bug using some piece of
> software.
> I don't really know where to begin, but I'm able to crash some applications
> via the application's menu, only with Cygwin/X (no crash with Xming, and no
> crash under any Linux distributions).
> For information, It goes through VirtualGL. I'll give the example on how to
> reproduce it with a free software : Paraview (under linux).
> So you open Paraview, then clic on "View" > show the "Toolbars" submenu,
> then you move your mouse directly on the "Sources" or "Edit" menu, then it
> crashes immediately. The only error message in the linux shell (from where
> it was launched) is "paraview: Fatal IO error: client killed".
> I got those lines in the XWin.0.log file :
> SetupSysMenu: GetSystemMenu() failed for HWND 0x130746
> winTopLevelWindowProc - WM_DESTROY - WM_WM_KILL

Thanks for reporting this problem, and the clear reproduction steps.

This is an interesting bug in the Cygwin X server's multiwindow mode. It 
seems that under some circumstances, when removing a WM_TRANSIENT_FOR 
window which in turn had a child WM_TRANSIENT_FOR window (e.g. a menu 
which has a sub-menu), the X server's integrated WM would get confused 
and think that the main window had been closed (so terminating the 

I've has a go at fixing this and uploaded a snapshot at [1].  Perhaps 
you could try that and see if it improves things for you?

(I guessed you are using 64-bit Cygwin. If you need a 32-bit build 
instead, please let me know and I'll generate one)


Jon Turney
Volunteer Cygwin/X X Server maintainer

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