将WM_CHANGEUISTATE发送到窗口应用程序后，当用鼠标访问菜单时，助记符不会显示在系统菜单上

下面的代码向窗口过程本身发送一个WM_CHANGEUISTATE，参数为：

LOWORD(wParam)=UIS_CLEAR

HIWORD(wParam)=UISF_HIDACCEL

lParam=0x0000000

当用鼠标左键单击窗口客户端区域时。

根据Raymond Chen的博客，当用鼠标访问菜单时，应该会显示系统菜单中的助记符。以下段落摘自本文：

清除标志将显示相应的指示符。例如，如果对于UISF_HIDE­FOCUS有一个UIS_CLEAR，这意味着您要显示焦点指标。

在我的例子中，我有一个UIS_CLEAR用于UISF_HIDACCEL，这意味着我想显示菜单快捷键。

如果您运行下面的代码并用鼠标左键单击应用程序客户端区域，则应使加速器在"系统"菜单中可见，即使使用鼠标访问此菜单也是如此。但这种情况不会发生，即，如果您左键单击窗口图标或右键单击窗口标题栏激活系统菜单，则系统菜单中的助记符将不会显示。我错过了什么？

#include <windows.h>
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR pszCmdLine, int nCmdShow)
{
WNDCLASSEX  wndclassx;
wchar_t szAppName[] = L"WM_CHANGEUISTATE";
wndclassx.cbSize = sizeof(WNDCLASSEX);
wndclassx.style = CS_HREDRAW | CS_VREDRAW;
wndclassx.lpfnWndProc = WndProc;
wndclassx.cbClsExtra = 0;
wndclassx.cbWndExtra = 0;
wndclassx.hInstance = hInstance;
wndclassx.hIcon = 0;
wndclassx.hCursor = LoadCursor(0, IDC_ARROW);
wndclassx.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
wndclassx.lpszClassName = szAppName;
wndclassx.lpszMenuName = nullptr;
wndclassx.hIconSm = 0;
if (!RegisterClassEx(&wndclassx)) return 0;
HWND hWnd = CreateWindow(szAppName, szAppName, WS_OVERLAPPEDWINDOW | WS_HSCROLL | WS_VSCROLL,
CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, 0, 0, hInstance, 0);
ShowWindow(hWnd, SW_MAXIMIZE);
UpdateWindow(hWnd);
MSG msg;
while (GetMessage(&msg, 0, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return (int)msg.wParam;
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
switch (message)
{
case WM_LBUTTONUP:
{
BOOL b;
// Note that in my system (Windows 10) I have:
//
// Control Panel > Ease of Access > Ease of Access Center > Make the keyboard easier
//
// and the option "Underline keyboard shortcuts and access keys" unmarked (the default). Therefore, the value
// returned in b below will be 0x00000000 (FALSE).
SystemParametersInfo(SPI_GETKEYBOARDCUES, 0, &b, 0);
// If b = FALSE, the SendMessage() below should underline the accelerators in the System menu, when this menu is
// accessed with the mouse. But that doesn't work. Why?
if( !b ) SendMessage(hwnd, WM_CHANGEUISTATE, MAKEWPARAM(UIS_CLEAR, UISF_HIDEACCEL), NULL);
}
break;
// Setting a break in the return below, one can see that WM_CHANGEUISTATE message is being sent to the window and
// passed to DefWindowProc().
case WM_CHANGEUISTATE:
return DefWindowProc(hwnd, message, wParam, lParam);

case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hwnd, message, wParam, lParam);
}
return 0;
}