Make wxSafeShowMessage() msgbox app-modal in some cases

Although we don't want to use MB_TASKMODAL unconditionally, because it
results in non-optimal UI behaviour when there is a parent window, there
is no reason not to use it when we don't have any parent anyhow, so do
this, at least.

src/common/log.cpp

@@ -184,8 +184,28 @@ void wxSafeShowMessage(const wxString& title, const wxString& text)
 {
 #ifdef __WINDOWS__
     const wxAppTraits* const traits = wxApp::GetTraitsIfExists();
-    ::MessageBox(traits ? traits->GetMainHWND() : NULL,
-                 text.t_str(), title.t_str(), MB_OK | MB_ICONSTOP);
+    const HWND hwndParent = traits ? traits->GetMainHWND() : NULL;
+    int flags = MB_OK | MB_ICONSTOP;
+
+    // Using MB_TASKMODAL with valid parent doesn't work well because it
+    // prevents the typical behaviour of modal message boxes, e.g. the message
+    // box doesn't come up to front when the parent is clicked. But if we don't
+    // have any parent anyhow, we can just as well use it, as we don't lose
+    // anything and it has a useful side effect of disabling any existing TLWs
+    // if there are any.
+    //
+    // Note that we also might have chosen to always use MB_TASKMODAL and NULL
+    // parent. This would have the advantage of always disabling all the window
+    // which, but at the cost of the behaviour mentioned above and other
+    // related problems, e.g. showing ugly default icon in Alt-Tab list and an
+    // extra taskbar button for the message box, so we don't do this, although
+    // perhaps we still should, at least in case when there is more than one
+    // TLW (but we can't check for this easily as this is non-GUI code and
+    // wxTopLevelWindows is not accessible from it).
+    if ( !hwndParent )
+        flags |= MB_TASKMODAL;
+
+    ::MessageBox(hwndParent, text.t_str(), title.t_str(), flags);
 #else
     wxFprintf(stderr, wxS("%s: %s\n"), title.c_str(), text.c_str());
     fflush(stderr);