c++ – Unwanted border around the window

I have a very simple wxFrame with wxButton inside it:

I created it like below:

...
public:
    MyFrame(wxWindow* ptr, int id, wxString name, wxPoint pos, wxSize size) : wxFrame(ptr, id, name, pos, size, wxBORDER_NONE)
    {
        ...
        HelloWorld = new wxButton(this, wxID_HIGHEST + 1, _T("Hello World"),
        wxDefaultPosition, wxDefaultSize, 0); 
        ...
    }

I’m still getting this thin grey borders around the button (seen next to the left and up button sides) even if I set wxBORDER_NONE for the parent frame!

Why is that unwanted border there, and HOW do I get rid of it completely?

P.S: The attached image is, of course, enlarged for better visibility.