Example of using `wxSplitterWindow`

splitter.cpp

#include <wx/app.h>
#include <wx/frame.h>
#include <wx/splitter.h>
#include <wx/textctrl.h>

class TextCtrlShowingSize : public wxTextCtrl {
public:
    explicit TextCtrlShowingSize(wxWindow* parent) : wxTextCtrl(parent, wxID_ANY, {}) {
        Bind(wxEVT_SIZE, [this](wxSizeEvent& event) {
            auto size = event.GetSize();
            SetValue(wxString::Format("Size: %d x %d", size.x, size.y));
            event.Skip();
        });
    }
};

class TestApp : public wxApp {
public:
    bool OnInit() override {
        auto f = new wxFrame(nullptr, wxID_ANY, "Window with a splitter");
        auto s = new wxSplitterWindow(f);
        s->SplitVertically(new TextCtrlShowingSize(s), new TextCtrlShowingSize(s), f->FromDIP(200));
        s->SetSashGravity(0.5);
        s->SetMinimumPaneSize(4);

        f->SetClientSize(f->FromDIP(wxSize(500, 300)));

        f->Show();
        return true;
    }
};

wxIMPLEMENT_APP(TestApp);