Manually hide our DesktopWindowXamlSource (#15165)

As discussed in #6507

Newer builds of Windows do this automatically. However, this was spotted
in the wild on 1.18. It's possible the threading changes created a
situation where the OS-side fix no longer applied to us. So let's just
do it manually. It doesn't have any side effects.

I saw this once on Win11, but couldn't repro it this morning when I
tried to add this fix. I'm just gonna assume this worked, despite the
fact that I can't repro it on win11 anymore.

closes #6507

See also #14957

## detailed description

> `WindowsXamlManager::XamlCore::Initialize` calls
`ConfigureCoreWindow`, which creates a `CoreWindow` on the thread

> Problem is, we're calling that on the main thread (which doesn't have
_any_ windows), and then eventually creating a `DesktopWindowXamlSource`
on a second thread for the actual window

> It's not that it "manages a window", it's that it "manages xaml on
Windows OS". just use ICoreWindowInterop -- QI for ICoreWindowInterop
and call get_WindowHandle.

Also see:
*
[ICoreWindowInterop](https://learn.microsoft.com/en-us/windows/win32/api/corewindow/nn-corewindow-icorewindowinterop)
*
[WindowsXamlManager.InitializeForCurrentThread](https://learn.microsoft.com/en-us/uwp/api/windows.ui.xaml.hosting.windowsxamlmanager.initializeforcurrentthread?view=winrt-22621#windows-ui-xaml-hosting-windowsxamlmanager-initializeforcurrentthread)
* The source code in
`onecoreuap\windows\dxaml\xcp\dxaml\lib\WindowsXamlManager_Partial.*`
* os.2020!6102020 which fixed MSFT:33498969, MSFT:27807465,
MSFT:21854264

src/cascadia/TerminalApp/App.cpp

@@ -4,6 +4,7 @@
 #include "pch.h"
 #include "App.h"
 #include "App.g.cpp"
+#include <CoreWindow.h>
 
 using namespace winrt;
 using namespace winrt::Windows::ApplicationModel::Activation;
@@ -32,6 +33,27 @@ namespace winrt::TerminalApp::implementation
         if (!dispatcherQueue)
         {
             _windowsXamlManager = xaml::Hosting::WindowsXamlManager::InitializeForCurrentThread();
+
+            // As of Process Model v3, terminal windows are all created on their
+            // own threads, but we still initiate XAML for the App on the main
+            // thread. Thing is, just initializing XAML creates a CoreWindow for
+            // us. On Windows 10, that CoreWindow will show up as a visible
+            // window on the taskbar, unless we hide it manually. So, go get it
+            // and do the SW_HIDE thing on it.
+            if (const auto& coreWindow{ winrt::Windows::UI::Core::CoreWindow::GetForCurrentThread() })
+            {
+                if (const auto& interop{ coreWindow.try_as<ICoreWindowInterop>() })
+                {
+                    HWND coreHandle{ 0 };
+                    interop->get_WindowHandle(&coreHandle);
+                    if (coreHandle)
+                    {
+                        // This prevents an empty "DesktopWindowXamlSource" from
+                        // appearing on the taskbar
+                        ShowWindow(coreHandle, SW_HIDE);
+                    }
+                }
+            }
         }
         else
         {

src/cascadia/WindowsTerminal/IslandWindow.cpp

@@ -307,6 +307,11 @@ void IslandWindow::Initialize()
     // stash the child interop handle so we can resize it when the main hwnd is resized
     interop->get_WindowHandle(&_interopWindowHandle);
 
+    // Immediately hide our XAML island hwnd. On earlier versions of Windows,
+    // this HWND could sometimes appear as an actual window in the taskbar
+    // without this!
+    ShowWindow(_interopWindowHandle, SW_HIDE);
+
     _rootGrid = winrt::Windows::UI::Xaml::Controls::Grid();
     _source.Content(_rootGrid);
 

src/cascadia/WindowsTerminal/IslandWindow.h

@@ -94,8 +94,8 @@ protected:
 
     HWND _interopWindowHandle;
 
-    winrt::Windows::UI::Xaml::Hosting::DesktopWindowXamlSource _source;
-    winrt::Windows::UI::Xaml::Controls::Grid _rootGrid;
+    winrt::Windows::UI::Xaml::Hosting::DesktopWindowXamlSource _source; // nulled in ctor
+    winrt::Windows::UI::Xaml::Controls::Grid _rootGrid; // nulled in ctor
     wil::com_ptr<ITaskbarList3> _taskbar;
 
     std::function<void(const HWND, const til::rect&)> _pfnCreateCallback;