Why does NativeWindow.De buggableCallbac k() get called from Debug.WriteLine ()?

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Hi there, people!

I m working on a c# control. Using VS2003 and writing for .NET 1.1. I
wrote an app that automaticly opens and closes (after a random Sleep)
forms that own one instance of my control each. While running my
application I'm seeing strange stacks like this:

system.windows. forms.dll!
ControlNativeWi ndow.WndProc(Sy stem.Windows.Fo rms.Message m =
{System.Windows .Forms.Message} ) + 0xda bytes
system.windows. forms.dll!
System.Windows. Forms.NativeWin dow.DebuggableC allback(int hWnd =
4196340, int msg = 16, int wparam = 0, int lparam = 0) + 0x39 bytes
system.dll!Syst em.Diagnostics. TraceInternal.W riteLine(string message =
"blablabla" ) + 0x4d bytes
system.dll!Syst em.Diagnostics. Debug.WriteLine (string message =
"blablabla" ) + 0xf bytes

As you see I'm sending a string to the VS output window. BUT! Before
exitting from "Debug.WriteLin e" the program decides to go and process
another message. Just how can processing of WM_CLOSE be a part of
Debug.WriteLine ?

I looked in Reflector and if the only Listener in Debug.Listenere s
collection is DefaultListener then there is no code in
Debug.WriteLine , except for a call to OutputDebugStri ng.

I hope this some well known feature of WinForms/Win API that I am not
aware of. Please give me a few URLs where I could read about this
stuff.
Thank you.