Add SDL_HINT_WINDOWS_ENABLE_MENU_MNEMONICS.

(cherry picked from commit 232ed540dbf3924543ab1a084a43f2e70f4b31a9)

include/SDL3/SDL_hints.h

@@ -1941,6 +1941,28 @@ extern "C" {
  */
 #define SDL_HINT_WINDOWS_DISABLE_THREAD_NAMING "SDL_WINDOWS_DISABLE_THREAD_NAMING"
 
+/**
+ *  \brief Controls whether menus can be opened with their keyboard shortcut (Alt+mnemonic).
+ *
+ *  If the mnemonics are enabled, then menus can be opened by pressing the Alt
+ *  key and the corresponding mnemonic (for example, Alt+F opens the File menu).
+ *  However, in case an invalid mnemonic is pressed, Windows makes an audible
+ *  beep to convey that nothing happened. This is true even if the window has
+ *  no menu at all!
+ *
+ *  Because most SDL applications don't have menus, and some want to use the Alt
+ *  key for other purposes, SDL disables mnemonics (and the beeping) by default.
+ *
+ *  Note: This also affects keyboard events: with mnemonics enabled, when a
+ *  menu is opened from the keyboard, you will not receive a KEYUP event for
+ *  the mnemonic key, and *might* not receive one for Alt.
+ *
+ *  This variable can be set to the following values:
+ *    "0"       - Alt+mnemonic does nothing, no beeping. (default)
+ *    "1"       - Alt+mnemonic opens menus, invalid mnemonics produce a beep.
+ */
+#define SDL_HINT_WINDOWS_ENABLE_MENU_MNEMONICS "SDL_WINDOWS_ENABLE_MENU_MNEMONICS"
+
 /**
  *  \brief  A variable controlling whether the windows message loop is processed by SDL 
  *

src/video/windows/SDL_windowsevents.c

@@ -1357,8 +1357,10 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 
     case WM_SYSCOMMAND:
     {
-        if ((wParam & 0xFFF0) == SC_KEYMENU) {
-            return 0;
+        if (!g_WindowsEnableMenuMnemonics) {
+            if ((wParam & 0xFFF0) == SC_KEYMENU) {
+                return 0;
+            }
         }
 
 #if defined(SC_SCREENSAVE) || defined(SC_MONITORPOWER)

src/video/windows/SDL_windowsvideo.c

@@ -24,6 +24,7 @@
 
 #include "../SDL_sysvideo.h"
 #include "../SDL_pixels_c.h"
+#include "../../SDL_hints_c.h"
 
 #include "SDL_windowsvideo.h"
 #include "SDL_windowsframebuffer.h"
@@ -38,24 +39,22 @@ static void WIN_VideoQuit(_THIS);
 
 /* Hints */
 SDL_bool g_WindowsEnableMessageLoop = SDL_TRUE;
+SDL_bool g_WindowsEnableMenuMnemonics = SDL_FALSE;
 SDL_bool g_WindowFrameUsableWhileCursorHidden = SDL_TRUE;
 
 static void SDLCALL UpdateWindowsEnableMessageLoop(void *userdata, const char *name, const char *oldValue, const char *newValue)
 {
-    if (newValue && *newValue == '0') {
-        g_WindowsEnableMessageLoop = SDL_FALSE;
-    } else {
-        g_WindowsEnableMessageLoop = SDL_TRUE;
-    }
+    g_WindowsEnableMessageLoop = SDL_GetStringBoolean(newValue, SDL_TRUE);
+}
+
+static void SDLCALL UpdateWindowsEnableMenuMnemonics(void *userdata, const char *name, const char *oldValue, const char *newValue)
+{
+    g_WindowsEnableMenuMnemonics = SDL_GetStringBoolean(newValue, SDL_FALSE);
 }
 
 static void SDLCALL UpdateWindowFrameUsableWhileCursorHidden(void *userdata, const char *name, const char *oldValue, const char *newValue)
 {
-    if (newValue && *newValue == '0') {
-        g_WindowFrameUsableWhileCursorHidden = SDL_FALSE;
-    } else {
-        g_WindowFrameUsableWhileCursorHidden = SDL_TRUE;
-    }
+    g_WindowFrameUsableWhileCursorHidden = SDL_GetStringBoolean(newValue, SDL_TRUE);
 }
 
 #if !defined(__XBOXONE__) && !defined(__XBOXSERIES__)
@@ -450,6 +449,7 @@ int WIN_VideoInit(_THIS)
 #endif
 
     SDL_AddHintCallback(SDL_HINT_WINDOWS_ENABLE_MESSAGELOOP, UpdateWindowsEnableMessageLoop, NULL);
+    SDL_AddHintCallback(SDL_HINT_WINDOWS_ENABLE_MENU_MNEMONICS, UpdateWindowsEnableMenuMnemonics, NULL);
     SDL_AddHintCallback(SDL_HINT_WINDOW_FRAME_USABLE_WHILE_CURSOR_HIDDEN, UpdateWindowFrameUsableWhileCursorHidden, NULL);
 
 #if !defined(__XBOXONE__) && !defined(__XBOXSERIES__)

src/video/windows/SDL_windowsvideo.h

@@ -460,6 +460,7 @@ typedef struct SDL_VideoData
 } SDL_VideoData;
 
 extern SDL_bool g_WindowsEnableMessageLoop;
+extern SDL_bool g_WindowsEnableMenuMnemonics;
 extern SDL_bool g_WindowFrameUsableWhileCursorHidden;
 
 typedef struct IDirect3D9 IDirect3D9;