Strange problem when performed window message processing in NPAPI plig-in under Windows.

I have NPAPI based plug-in which called from JS through
plug-in scriptable object.

I report that plug-in is windowed (but seens browser do not check this)
through own 'NPP_GetValue' func.

In function that invoked on JS call I show native Windows window
which I create without passing parent main browser window
(by CreateWindowEx).

My window message processing loop:

     while (1) {
         MSG msg;
         BOOL fGotMessage = GetMessage(&msg, NULL, 0, 0);
         if (fGotMessage == 0)
             break;
         if (fGotMessage == -1) {
             MessageBox(NULL, L"GetMessage", errorMsg, MB_ICONERROR);
             break;
         }
         if (!IsDialogMessage(hWnd, &msg)) {
             TranslateMessage(&msg);
             DispatchMessage(&msg);
         }
     }

I found that Firefox, Opera, Chrome and Safary have strange
behaviour after invoking by plug-in func from JS (and closing window 
showed by plug-in) like clikcing on some URL do not perform
jumping to this URL or ever browser crashing (with WinDbg I check
that crash occur outside my plug-in code).

I found that this issue related to window message processing
when some message passed to 'DefWindowProc' in my 'WndProc'.

If I put:

     if (message >= 0xc000)
         return -1;

before switch in:

WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
     switch (message) {
     case WM_CREATE: {
           ....

Firefox, Chrome and Opera browsers work fine. Safary still crashes.
Without this 'if (message >= 0xc000)' all browser have described
problems.

I experiment with different values in '(message >= 0xc000)' like
WM_USER/WM_APP/0xc000 and log all messages but reading log
does not take me some thing valuable.

Note that I am have a few skills in Windows development.
I found that working with WinDbg is hard to understand what happen.

Any suggestions please!

-- 
Best regards!