Question

How determine if mouse points to(hover on) maximise button of window even if this window is not of my application.
Is there API for that ?

Answer 1

You may send a WM_NCHITTEST to that window. The return value will correspond to the object type on the requested coordinates.

Something like this:

bool IsMouseOverMaxBtn(HWND hWnd)
{
    POINT pt;
    VERIFY(GetCursorPos(&pt)); // get mouse position

    int retVal = SendMessage(hWnd, WM_NCHITTEST, 0, MAKELONG(pt.x, pt.y));

    return HTMAXBUTTON == retVal;
}

Edit:

You may send this message to any window (not necessarily belong to your thread/process). Since no pointers are involved (such as string pointers) – there’s no problem.

However you should note that sending (not posting) a message to a window belonging to another thread is a pretty heavy operation, during which your thread is suspended. There may even happen a situation where your thread hangs, because the thread of the application that serves that window hangs.

You may consider using SendMessageTimeout to guarantee your thread won’t hang.