Win32 ShellExecute Clears Message Queue?

main.c

// Compile from Visual Studio Command Prompt with:
//
//     cl main.c
//
#pragma comment(lib, "user32.lib")
#pragma comment(lib, "shell32.lib")

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

static unsigned DropMessages()
{
    unsigned count = 0;
    MSG msg;
    while (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE)) {
        printf("    droping message %d\n", msg.message);
        count++;
    }
    if (count == 0) {
        printf("    empty (no messages were dropped)\n");
    }
    return count;
}

// NOTE: using wWinMain doesn't help
//int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, WCHAR *pCmdLine, int nCmdShow)
int main()
{
    //
    // Show that the message queue starts out empty
    //
    printf("dropping messages, it should start out empty:\n");
    {
        unsigned dropped = DropMessages();
        if (dropped != 0) abort();
    }

    //
    // Post a message to the queue and show that it's in there
    //
    printf("posting message\n");
    if (!PostThreadMessage(GetCurrentThreadId(), WM_USER, 0, 0)) {
        printf("PostThreadMessage failed, error={}\n", GetLastError());
        return -1;
    }
    printf("dropping messages, it should drop 1:\n");
    {
        unsigned dropped = DropMessages();
        if (dropped != 1) abort();
    }
    
    //
    // Post a message to the queue, call ShellExecute which clears the queue!?!
    //
    printf("posting message\n");
    if (!PostThreadMessage(GetCurrentThreadId(), WM_USER, 0, 0)) {
        printf("PostThreadMessage failed, error={}\n", GetLastError());
        return -1;
    }

    static const int enable_bug = 1;
    if (enable_bug) {
        printf("calling ShellExecute, this clears the message queue!?!\n");
        INT_PTR result = (INT_PTR)ShellExecute(NULL, NULL, NULL, NULL, NULL, 0);
        if (result <= 32) {
            printf("ShellExecute failed, result={}, error={}\n", result, GetLastError());
            return -1;
        }
    }
    
    printf("dropping messages, you'd think it should have 1 but:\n");
    {
        unsigned dropped = DropMessages();
        if (dropped == 1) {
            printf("Success!\n");
        } else {
            printf("Error: expected to drop exactly 1 message but dropped %d\n", dropped);
        }

    }
    return 0;
}