wxFrame does not process Tab button - c++

Question

I have a simple wxFrame with 3 buttons. After I press Tab nothing happens. In the forum I found that wxFrame should process Tab button events normally and switch focus between controls. I tried with wxTAB_TRAVERSAL and without it, but look like no result.
Here is my code. wxWidgets 3.0.2. Please, help.
class TabWnd
: public wxFrame
{
public:
TabWnd()
: wxFrame(nullptr,
wxID_ANY,
wxEmptyString,
wxDefaultPosition,
wxDefaultSize,
wxDEFAULT_FRAME_STYLE | wxTAB_TRAVERSAL)
{
wxBoxSizer* sz = new wxBoxSizer(wxVERTICAL);
wxButton* b1 = new wxButton(this, wxID_ANY, wxT("First"));
sz->Add(b1, 0, wxALL, 5);
wxButton* b2 = new wxButton(this, wxID_ANY, wxT("Second"));
sz->Add(b2, 0, wxALL, 5);
wxButton* b3 = new wxButton(this, wxID_ANY, wxT("Third"));
sz->Add(b3, 0, wxALL, 5);
SetSizer(sz);
Layout();
Centre(wxBOTH);
}
};
class WxguiApp
: public wxApp
{
public:
bool OnInit() override
{
TabWnd* mainWnd = new TabWnd();
mainWnd->Show();
SetTopWindow(mainWnd);
return true;
}
};
IMPLEMENT_APP(WxguiApp);

Answer 1

Try adding a panel between the frame and the buttons like this:
wxBoxSizer* sz = new wxBoxSizer(wxVERTICAL);
wxPanel* pnl = new wxPanel( this, wxID_ANY );
wxBoxSizer* sz2 = new wxBoxSizer( wxVERTICAL );
wxButton* b1 = new wxButton(pnl, wxID_ANY, wxT("First"));
sz2->Add(b1, 0, wxALL, 5);
wxButton* b2 = new wxButton(pnl, wxID_ANY, wxT("Second"));
sz2->Add(b2, 0, wxALL, 5);
wxButton* b3 = new wxButton(pnl, wxID_ANY, wxT("Third"));
sz2->Add(b3, 0, wxALL, 5);
pnl->SetSizer( sz2 );
sz->Add( pnl, 1, wxEXPAND );
SetSizer(sz);
Layout();
Centre(wxBOTH);