Use standard 1px window borders on NC Island Window (#3394)

We take the standard window frame except that we remove the top part
(see `NonClientIslandWindow::_OnNcCalcSize`), then we put little 1 pixel
wide top border back in the client area using
`DwmExtendFrameIntoClientArea` and then we put the XAML island and the
drag bar on top.

Most of this PR is comments to explain how the code works and also
removing complex code that was needed to handle the weird cases when the
borders were custom.

I've also refactored a little bit the `NonClientIslandWindow` class.

* Fix DwmExtendFrameIntoClientArea values
* Fix WM_NCHITTEST handling
* Position the XAML island window correctly
* Fix weird colors in drag bar and hide old title bar buttons
* Fix the window's position when maximized
* Add support for dark theme on the frame
* DRY shared code between conhost and new terminal
* Fix drag bar and remove dead code
* Remove dead code and use cached DPI
* Refactor code
* Remove impossible TODO
* Use system metrics instead of hardcoding resize border height
* Use theme from app settings instead of system theme. Improve comments. Remove unused DWM frame on maximize.
* Fix initial position DPI handling bug and apply review changes
* Fix thick borders with DPI > 96

Closes #3064.
Closes #1307.
Closes #3136.
Closes #1897.
Closes #3222.
Closes #1859.

src/cascadia/TerminalApp/App.cpp

@@ -335,7 +335,8 @@ namespace winrt::TerminalApp::implementation
         TerminalSettings settings = _settings->MakeSettings(std::nullopt);
 
         // TODO MSFT:21150597 - If the global setting "Always show tab bar" is
-        // set, then we'll need to add the height of the tab bar here.
+        // set or if "Show tabs in title bar" is set, then we'll need to add
+        // the height of the tab bar here.
 
         return TermControl::GetProposedDimensions(settings, dpi);
     }
@@ -394,6 +395,17 @@ namespace winrt::TerminalApp::implementation
         return point;
     }
 
+    winrt::Windows::UI::Xaml::ElementTheme App::GetRequestedTheme()
+    {
+        if (!_loadedInitialSettings)
+        {
+            // Load settings if we haven't already
+            LoadSettings();
+        }
+
+        return _settings->GlobalSettings().GetRequestedTheme();
+    }
+
     bool App::GetShowTabsInTitlebar()
     {
         if (!_loadedInitialSettings)

src/cascadia/TerminalApp/App.h

@@ -32,6 +32,7 @@ namespace winrt::TerminalApp::implementation
 
         Windows::Foundation::Point GetLaunchDimensions(uint32_t dpi);
         winrt::Windows::Foundation::Point GetLaunchInitialPositions(int32_t defaultInitialX, int32_t defaultInitialY);
+        winrt::Windows::UI::Xaml::ElementTheme GetRequestedTheme();
         LaunchMode GetLaunchMode();
         bool GetShowTabsInTitlebar();
 

src/cascadia/TerminalApp/App.idl

@@ -31,6 +31,7 @@ namespace TerminalApp
 
         Windows.Foundation.Point GetLaunchDimensions(UInt32 dpi);
         Windows.Foundation.Point GetLaunchInitialPositions(Int32 defaultInitialX, Int32 defaultInitialY);
+        Windows.UI.Xaml.ElementTheme GetRequestedTheme();
         LaunchMode GetLaunchMode();
         Boolean GetShowTabsInTitlebar();
         void TitlebarClicked();

src/cascadia/WindowsTerminal/AppHost.cpp

@@ -22,7 +22,7 @@ AppHost::AppHost() noexcept :
 
     if (_useNonClientArea)
     {
-        _window = std::make_unique<NonClientIslandWindow>();
+        _window = std::make_unique<NonClientIslandWindow>(_app.GetRequestedTheme());
     }
     else
     {
@@ -188,54 +188,25 @@ void AppHost::_HandleCreateWindow(const HWND hwnd, RECT proposedRect, winrt::Ter
 
         auto initialSize = _app.GetLaunchDimensions(dpix);
 
-        const short _currentWidth = Utils::ClampToShortMax(
+        const short islandWidth = Utils::ClampToShortMax(
             static_cast<long>(ceil(initialSize.X)), 1);
-        const short _currentHeight = Utils::ClampToShortMax(
+        const short islandHeight = Utils::ClampToShortMax(
             static_cast<long>(ceil(initialSize.Y)), 1);
 
-        // Create a RECT from our requested client size
-        auto nonClient = Viewport::FromDimensions({ _currentWidth,
-                                                    _currentHeight })
-                             .ToRect();
+        RECT islandFrame = {};
+        bool succeeded = AdjustWindowRectExForDpi(&islandFrame, WS_OVERLAPPEDWINDOW, false, 0, dpix);
+        // If we failed to get the correct window size for whatever reason, log
+        // the error and go on. We'll use whatever the control proposed as the
+        // size of our window, which will be at least close.
+        LOG_LAST_ERROR_IF(!succeeded);
 
-        // Get the size of a window we'd need to host that client rect. This will
-        // add the titlebar space.
         if (_useNonClientArea)
         {
-            // If we're in NC tabs mode, do the math ourselves. Get the margins
-            // we're using for the window - this will include the size of the
-            // titlebar content.
-            const auto pNcWindow = static_cast<NonClientIslandWindow*>(_window.get());
-            const MARGINS margins = pNcWindow->GetFrameMargins();
-            nonClient.left = 0;
-            nonClient.top = 0;
-            nonClient.right = margins.cxLeftWidth + nonClient.right + margins.cxRightWidth;
-            nonClient.bottom = margins.cyTopHeight + nonClient.bottom + margins.cyBottomHeight;
-        }
-        else
-        {
-            bool succeeded = AdjustWindowRectExForDpi(&nonClient, WS_OVERLAPPEDWINDOW, false, 0, dpix);
-            if (!succeeded)
-            {
-                // If we failed to get the correct window size for whatever reason, log
-                // the error and go on. We'll use whatever the control proposed as the
-                // size of our window, which will be at least close.
-                LOG_LAST_ERROR();
-                nonClient = Viewport::FromDimensions({ _currentWidth,
-                                                       _currentHeight })
-                                .ToRect();
-            }
-
-            // For client island scenario, there is an invisible border of 8 pixels.
-            // We need to remove this border to guarantee the left edge of the window
-            // coincides with the screen
-            const auto pCWindow = static_cast<IslandWindow*>(_window.get());
-            const RECT frame = pCWindow->GetFrameBorderMargins(dpix);
-            proposedRect.left += frame.left;
+            islandFrame.top = -NonClientIslandWindow::topBorderVisibleHeight;
         }
 
-        adjustedHeight = nonClient.bottom - nonClient.top;
-        adjustedWidth = nonClient.right - nonClient.left;
+        adjustedWidth = -islandFrame.left + islandWidth + islandFrame.right;
+        adjustedHeight = -islandFrame.top + islandHeight + islandFrame.bottom;
     }
 
     const COORD origin{ gsl::narrow<short>(proposedRect.left),
@@ -294,5 +265,5 @@ void AppHost::_UpdateTitleBarContent(const winrt::Windows::Foundation::IInspecta
 // - <none>
 void AppHost::_UpdateTheme(const winrt::TerminalApp::App&, const winrt::Windows::UI::Xaml::ElementTheme& arg)
 {
-    _window->UpdateTheme(arg);
+    _window->OnApplicationThemeChanged(arg);
 }

src/cascadia/WindowsTerminal/BaseWindow.h

@@ -34,10 +34,8 @@ class BaseWindow
             WINRT_ASSERT(that);
             WINRT_ASSERT(!that->_window);
             that->_window = wil::unique_hwnd(window);
-            SetWindowLongPtr(window, GWLP_USERDATA, reinterpret_cast<LONG_PTR>(that));
 
-            EnableNonClientDpiScaling(window);
-            that->_currentDpi = GetDpiForWindow(window);
+            return that->_OnNcCreate(wparam, lparam);
         }
         else if (T* that = GetThisFromHandle(window))
         {
@@ -245,6 +243,20 @@ class BaseWindow
     std::wstring _title = L"";
 
     bool _minimized = false;
+
+    // Method Description:
+    // - This method is called when the window receives the WM_NCCREATE message.
+    // Return Value:
+    // - The value returned from the window proc.
+    virtual [[nodiscard]] LRESULT _OnNcCreate(WPARAM wParam, LPARAM lParam) noexcept
+    {
+        SetWindowLongPtr(_window.get(), GWLP_USERDATA, reinterpret_cast<LONG_PTR>(this));
+
+        EnableNonClientDpiScaling(_window.get());
+        _currentDpi = GetDpiForWindow(_window.get());
+
+        return DefWindowProc(_window.get(), WM_NCCREATE, wParam, lParam);
+    };
 };
 
 template<typename T>

src/cascadia/WindowsTerminal/IslandWindow.cpp

@@ -243,15 +243,6 @@ IRawElementProviderSimple* IslandWindow::_GetUiaProvider()
     return _pUiaProvider;
 }
 
-RECT IslandWindow::GetFrameBorderMargins(unsigned int currentDpi)
-{
-    const auto windowStyle = GetWindowStyle(_window.get());
-    const auto targetStyle = windowStyle & ~WS_DLGFRAME;
-    RECT frame{};
-    AdjustWindowRectExForDpi(&frame, targetStyle, false, GetWindowExStyle(_window.get()), currentDpi);
-    return frame;
-}
-
 // Method Description:
 // - Called when the window has been resized (or maximized)
 // Arguments:
@@ -300,7 +291,7 @@ void IslandWindow::OnAppInitialized()
 // - arg: the ElementTheme to use as the new theme for the UI
 // Return Value:
 // - <none>
-void IslandWindow::UpdateTheme(const winrt::Windows::UI::Xaml::ElementTheme& requestedTheme)
+void IslandWindow::OnApplicationThemeChanged(const winrt::Windows::UI::Xaml::ElementTheme& requestedTheme)
 {
     _rootGrid.RequestedTheme(requestedTheme);
     // Invalidate the window rect, so that we'll repaint any elements we're

src/cascadia/WindowsTerminal/IslandWindow.h

@@ -23,19 +23,17 @@ class IslandWindow :
 
     [[nodiscard]] virtual LRESULT MessageHandler(UINT const message, WPARAM const wparam, LPARAM const lparam) noexcept override;
     IRawElementProviderSimple* _GetUiaProvider();
-    RECT GetFrameBorderMargins(unsigned int currentDpi);
     void OnResize(const UINT width, const UINT height) override;
     void OnMinimize() override;
     void OnRestore() override;
     virtual void OnAppInitialized();
     virtual void SetContent(winrt::Windows::UI::Xaml::UIElement content);
+    virtual void OnApplicationThemeChanged(const winrt::Windows::UI::Xaml::ElementTheme& requestedTheme);
 
     virtual void Initialize();
 
     void SetCreateCallback(std::function<void(const HWND, const RECT, winrt::TerminalApp::LaunchMode& launchMode)> pfn) noexcept;
 
-    void UpdateTheme(const winrt::Windows::UI::Xaml::ElementTheme& requestedTheme);
-
 #pragma region IUiaWindow
     void ChangeViewport(const SMALL_RECT /*NewWindow*/)
     {