Bug 1774694: Function, browser, media and volume keys should not hide cursor on Windows r=cmartin

Also includes the Fn key, which isn't documented to map to a special virtual key code but maps to 0xff, an undefined code, on tested machines.

Differential Revision: https://phabricator.services.mozilla.com/D149928

widget/windows/nsWindow.cpp

@@ -671,11 +671,16 @@ static bool IsMouseVanishKey(WPARAM aVirtKey) {
     case VK_RIGHT:
     case VK_PRIOR:  // PgUp
     case VK_NEXT:   // PgDn
+    case 0xff:      // Undefined.  May be sent for Fn key.
       return false;
     default:
-      // Return true unless Ctrl or Alt is pressed
+      // Vanish unless Ctrl or Alt is also pressed, or if a key in
+      // a relevant range is pressed.
+      // The range between VK_F1 and VK_LAUNCH_APP2 includes control,
+      // function, browser, volume and media keys, all of which we ignore.
       return (GetKeyState(VK_CONTROL) & 0x8000) != 0x8000 &&
-             (GetKeyState(VK_MENU) & 0x8000) != 0x8000;
+             (GetKeyState(VK_MENU) & 0x8000) != 0x8000 &&
+             (aVirtKey < VK_F1 || aVirtKey > VK_LAUNCH_APP2);
   }
 }