SetWindowPos not setting Z order properly

Question

I am working on a window manager. Here is some code to stack windows diagonally:

void stack_windows_diagonal(HWND* windows, int width, int height){
    if(windows == NULL) return;

    int size = 0;
    for(;windows[size]; size++);

    RECT scr_dim;
    scr_dim.left   = GetSystemMetrics(SM_XVIRTUALSCREEN);
    scr_dim.top    = GetSystemMetrics(SM_YVIRTUALSCREEN);
    scr_dim.right  = GetSystemMetrics(SM_CXVIRTUALSCREEN);
    scr_dim.bottom = GetSystemMetrics(SM_CYVIRTUALSCREEN);

    int xoffset = 40;
    int yoffset = 40;

    int startx = (scr_dim.right - width) / 2 - (((size - 1) * xoffset) / 2);
    int starty = (scr_dim.bottom - height) / 2 - (((size - 1) * yoffset) / 2);

    for(int i = size - 1; i >= 0; i--){
        bool success = SetWindowPos(windows[i], i == size - 1? HWND_TOP : windows[i - 1], startx + xoffset * i, starty + yoffset * i, width, height, 0);
        if(!success) {
            DWORD error = GetLastError();
            fprintf(stderr, "Failed to reposition window. Error code: %lu\n", error);
            continue;
        }

        if(i < size - 1)
            continue;

        success = SetForegroundWindow(windows[i]);
        if(!success){
            DWORD error = GetLastError();
            if(GetLastError())
                fprintf(stderr, "Failed to bring window to top. Error code: %lu\n", error);
        }

    }

    free(windows);

}

windows is terminated by NULL. Currently, this is only used for managing file explorer windows. The appear at the right x y coordinates, but the Z order is often very wrong. I get the same result setting all windows to HWND_TOP. The order they appear in is sometimes correct, but it seems very random. Why is this?

EDIT: Image showing the issue:

Answer 1

As Jonathan Potter said, window positioning is asynchronous.

Putting a Sleep() in after the call to SetWindowPos() works.