Demonstration of button border problem Windows API

common_controls.manifest.xml

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly>
  <dependency>
    <dependentAssembly>
        <assemblyIdentity
            type="win32"
            name="Microsoft.Windows.Common-Controls"
            version="6.0.0.0"
            processorArchitecture="*"
            publicKeyToken="6595b64144ccf1df"
            language="*"
        />
    </dependentAssembly>
  </dependency>
</assembly>

main.cpp

#include <assert.h>
#include <stdexcept>
#include <string>
using namespace std;

bool hopefully( bool const c ) { return c; }
bool throwX( string const& s ) { throw runtime_error( s ); }

#undef  UNICODE
#define UNICODE
#include <windows.h>

LRESULT CALLBACK windowProc(
    HWND const      window,
    UINT const      messageId,
    WPARAM const    wParam,
    LPARAM const    lParam
    )
{
    int const idStartButton = 100;
    switch( messageId )
    {
    case WM_CLOSE:
        DestroyWindow( window );
        break;
    case WM_CREATE:
        CreateWindowEx(
            WS_EX_TRANSPARENT, L"Button", L"Blah blah",
            WS_CHILD | WS_VISIBLE | BS_PUSHBUTTON, 20, 50, 75, 25,
            window,
            reinterpret_cast<HMENU>( idStartButton - 1 ),
            GetModuleHandle( nullptr ),
            nullptr
            );
        CreateWindowEx(
            0, L"Button", L"Start",
            WS_CHILD | WS_VISIBLE | BS_DEFPUSHBUTTON, 120, 50, 75, 25,
            window,
            reinterpret_cast<HMENU>( idStartButton ),
            GetModuleHandle( nullptr ),
            nullptr
            );
        break;
    case WM_DESTROY:
        PostQuitMessage( EXIT_SUCCESS );
        break;
    }
    return DefWindowProc( window, messageId, wParam, lParam );
}

void cppMain()
{
    WNDCLASS    classInfo   = {};
    classInfo.hbrBackground = reinterpret_cast<HBRUSH>( COLOR_WINDOW + 1 );
    classInfo.hCursor       = LoadCursor( 0, IDC_ARROW );
    classInfo.hIcon         = LoadIcon( 0, IDI_APPLICATION );
    classInfo.hInstance     = GetModuleHandle( nullptr );
    classInfo.lpfnWndProc   = &windowProc;
    classInfo.lpszClassName = L"MainWindow";
    classInfo.style         = 0;
    ATOM const classId = RegisterClass( &classInfo );
    hopefully( classId != 0 )
        || throwX( "RegisterClass failed" );
    HWND const window = CreateWindow(
        MAKEINTATOM( classId ),
        L"My main window",
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
        0,                          // parent
        0,                          // menu
        GetModuleHandle( nullptr ),
        0                           // lpParam
        );
    hopefully( window != 0 )
        || throwX( "CreateWindow failed" );
    ShowWindow( window, SW_SHOW );
    for( ;; )
    {
        MSG m;
        int const result = GetMessage( &m, 0, 0, 0 );
        hopefully( result >= 0 )
            || throwX( "GetMessage failed" );
        if( result == 0 )
        {
            assert( m.message == WM_QUIT );
            hopefully( m.wParam == EXIT_SUCCESS )
                || throwX( "WM_QUIT with non-zero exit code." );
            break;
        }
        TranslateMessage( &m );
        DispatchMessage( &m );
    }
}

#include <iostream>

int main()
{
    try
    {
        cppMain();
        return EXIT_SUCCESS;
    }
    catch( exception const& x )
    {
        cerr << "!" << x.what() << endl;
    }
    return EXIT_FAILURE;
}