Clear the system menu when we refrigerate a window (#16225)

As in the title. Also fixes a crash for refrigeration with the rainbow
border.

Closes #16211

Tested by manually forcing us into Windows 10 mode (to refrigerate the
window). That immediately repros the bug, which was simple enough to
fix.

src/cascadia/WindowsTerminal/AppHost.cpp

@@ -484,7 +484,7 @@ void AppHost::_revokeWindowCallbacks()
     // I suspect WinUI wouldn't like that very much. As such unregister all event handlers first.
     _revokers = {};
     _showHideWindowThrottler.reset();
-
+    _stopFrameTimer();
     _revokeWindowCallbacks();
 
     // DO NOT CLOSE THE WINDOW

src/cascadia/WindowsTerminal/IslandWindow.cpp

@@ -73,6 +73,8 @@ void IslandWindow::Refrigerate() noexcept
     // This pointer will get re-set in _warmInitialize
     SetWindowLongPtr(_window.get(), GWLP_USERDATA, 0);
 
+    _resetSystemMenu();
+
     _pfnCreateCallback = nullptr;
     _pfnSnapDimensionCallback = nullptr;
 
@@ -1917,6 +1919,12 @@ void IslandWindow::RemoveFromSystemMenu(const winrt::hstring& itemLabel)
     _systemMenuItems.erase(it->first);
 }
 
+void IslandWindow::_resetSystemMenu()
+{
+    // GetSystemMenu(..., true) will revert the menu to the default state.
+    GetSystemMenu(_window.get(), TRUE);
+}
+
 void IslandWindow::UseDarkTheme(const bool v)
 {
     const BOOL attribute = v ? TRUE : FALSE;

src/cascadia/WindowsTerminal/IslandWindow.h

@@ -159,6 +159,7 @@ class IslandWindow :
 
     std::unordered_map<UINT, SystemMenuItemInfo> _systemMenuItems;
     UINT _systemMenuNextItemId;
+    void _resetSystemMenu();
 
 private:
     // This minimum width allows for width the tabs fit