title: wxWidgets drawing failure
body: <p>im using wxWidgets to create a simple GUI, but somehow it doesnt show what i want.</p>
  <pre><code>Frame::Frame(std::string title)
      :   wxFrame(NULL, wxID_ANY, title)
  {
      constexpr uint32 c_margin = 10;
      
      wxListView* listView_fileDrop_1     = new wxListView(this);
      wxListView* listView_fileDrop_2     = new wxListView(this);
      
      wxPanel* panel_diagramComparison    = new wxPanel(this, wxID_ANY, wxDefaultPosition, wxSize(200, 200));
      wxPanel* panel_diagramLimit             = new wxPanel(this, wxID_ANY, wxDefaultPosition, wxSize(200, 200));
      
      panel_diagramComparison-&gt;SetBackgroundColour(   wxColor(0,      255,    0));
      panel_diagramLimit-&gt;SetBackgroundColour(            wxColor(0,      0,      255));
      
      
      
      wxBoxSizer* boxSizer_fileDrop = new wxBoxSizer(wxHORIZONTAL);
      boxSizer_fileDrop-&gt;Add(listView_fileDrop_1, 1, wxEXPAND,                        c_margin);
      boxSizer_fileDrop-&gt;Add(listView_fileDrop_2, 1, wxEXPAND | wxLEFT ,  c_margin);
      
      wxBoxSizer* boxSizer_window = new wxBoxSizer(wxVERTICAL);
      boxSizer_window-&gt;Add(boxSizer_fileDrop,             0, wxEXPAND | wxTOP | wxLEFT | wxRIGHT, c_margin);
      boxSizer_window-&gt;Add(panel_diagramComparison, 1, wxEXPAND | wxTOP | wxLEFT | wxRIGHT, c_margin);
      boxSizer_window-&gt;Add(panel_diagramLimit,            1, wxEXPAND | wxALL,                                        c_margin);
      
      this-&gt;SetSizer(boxSizer_window);
      Maximize();
  }
  </code></pre>
  <p>This is my Code so far, Frame is derived from wxFrame.
  But the Output doesnt look expected, see the following link (im not allowed to embed Images because im new...):
  <a href="https://ibb.co/TmCMwxG" rel="nofollow noreferrer">https://ibb.co/TmCMwxG</a></p>
  <p>What does the blue Rectangle in the top-left corner do there and how can i get it away?</p>
  <p>Thanks in advance</p>