Don't NCPAINT our window, PAINT our window (microsoft#1898)

* This definitely works for getting shadow, pointy corners back

  Don't do anything in NCPAINT. If you do, you have to do everything. But the
  whole point of DwmExtendFrameIntoClientArea is to let you paint the NC area in
  your normal paint. So just do that dummy.

  * This doesn't transition across monitors.
  * This has a window style change I think is wrong.
  * I'm not sure the margins change is important.

* The window style was _not_ important

* Still getting a black xaml islands area (the HRGN) when we switch to high DPI

* I don't know if this affects anything.

* heyo this works.

  I'm not entirely sure why. But if we only update the titlebar drag region when
  that actually changes, it's a _lot_ smoother. I'm not super happy with the
  duplicated work in _UpdateDragRegion and OnSize, but checking this in in case
  I can't figure that out.

* Add more comments and cleanup

* Some PR nits, fix the titlebar painting on maximize

src/cascadia/WindowsTerminal/BaseWindow.h

@@ -89,6 +89,7 @@ class BaseWindow
                 // do nothing.
                 break;
             }
+            break;
         }
         case CM_UPDATE_TITLE:
         {

src/cascadia/WindowsTerminal/NonClientIslandWindow.cpp

@@ -34,10 +34,17 @@ NonClientIslandWindow::~NonClientIslandWindow()
 {
 }
 
-void NonClientIslandWindow::OnDragBarSizeChanged(winrt::Windows::Foundation::IInspectable sender, winrt::Windows::UI::Xaml::SizeChangedEventArgs eventArgs)
+// Method Description:
+// - Called when the app's size changes. When that happens, the size of the drag
+//   bar may have changed. If it has, we'll need to update the WindowRgn of the
+//   interop window.
+// Arguments:
+// - <unused>
+// Return Value:
+// - <none>
+void NonClientIslandWindow::OnDragBarSizeChanged(winrt::Windows::Foundation::IInspectable /*sender*/, winrt::Windows::UI::Xaml::SizeChangedEventArgs /*eventArgs*/)
 {
-    InvalidateRect(NULL, NULL, TRUE);
-    ForceResize();
+    _UpdateDragRegion();
 }
 
 void NonClientIslandWindow::OnAppInitialized(winrt::TerminalApp::App app)
@@ -101,8 +108,6 @@ void NonClientIslandWindow::OnSize(const UINT width, const UINT height)
     const auto xPos = _isMaximized ? _maximizedMargins.cxLeftWidth : dragX;
     const auto yPos = _isMaximized ? _maximizedMargins.cyTopHeight : dragY;
 
-    winrt::check_bool(SetWindowPos(_interopWindowHandle, HWND_BOTTOM, xPos, yPos, windowsWidth, windowsHeight, SWP_SHOWWINDOW));
-
     if (_rootGrid)
     {
         winrt::Windows::Foundation::Size size{ (windowsWidth / scale) + 0.5f, (windowsHeight / scale) + 0.5f };
@@ -113,8 +118,53 @@ void NonClientIslandWindow::OnSize(const UINT width, const UINT height)
         _rootGrid.Arrange(finalRect);
     }
 
+    // I'm not sure that HWND_BOTTOM is any different than HWND_TOP for us.
+    winrt::check_bool(SetWindowPos(_interopWindowHandle, HWND_BOTTOM, xPos, yPos, windowsWidth, windowsHeight, SWP_SHOWWINDOW));
+}
+
+// Method Description:
+// - Update the region of our window that is the draggable area. This happens in
+//   response to a OnDragBarSizeChanged event. We'll calculate the areas of the
+//   window that we want to display XAML content in, and set the window region
+//   of our child xaml-island window to that region. That way, the parent window
+//   will still get NCHITTEST'ed _outside_ the XAML content area, for things
+//   like dragging and resizing.
+// Arguments:
+// - <none>
+// Return Value:
+// - <none>
+void NonClientIslandWindow::_UpdateDragRegion()
+{
     if (_dragBar)
     {
+        // TODO:GH#1897 This is largely duplicated from OnSize, and we should do
+        // better than that.
+        const auto windowRect = GetWindowRect();
+        const auto width = windowRect.right - windowRect.left;
+        const auto height = windowRect.bottom - windowRect.top;
+
+        const auto dpi = ::GetDpiForWindow(_window.get());
+
+        const auto dragY = ::GetSystemMetricsForDpi(SM_CYDRAG, dpi);
+        const auto dragX = ::GetSystemMetricsForDpi(SM_CXDRAG, dpi);
+
+        // If we're maximized, we don't want to use the frame as our margins,
+        // instead we want to use the margins from the maximization. If we included
+        // the left&right sides of the frame in this calculation while maximized,
+        // you' have a few pixels of the window border on the sides while maximized,
+        // which most apps do not have.
+        const auto bordersWidth = _isMaximized ?
+                                      (_maximizedMargins.cxLeftWidth + _maximizedMargins.cxRightWidth) :
+                                      (dragX * 2);
+        const auto bordersHeight = _isMaximized ?
+                                       (_maximizedMargins.cyBottomHeight + _maximizedMargins.cyTopHeight) :
+                                       (dragY * 2);
+
+        const auto windowsWidth = width - bordersWidth;
+        const auto windowsHeight = height - bordersHeight;
+        const auto xPos = _isMaximized ? _maximizedMargins.cxLeftWidth : dragX;
+        const auto yPos = _isMaximized ? _maximizedMargins.cyTopHeight : dragY;
+
         const auto dragBarRect = GetDragAreaRect();
         const auto nonClientHeight = dragBarRect.bottom - dragBarRect.top;
 
@@ -128,8 +178,6 @@ void NonClientIslandWindow::OnSize(const UINT width, const UINT height)
         winrt::check_bool(CombineRgn(_dragBarRegion.get(), nonClientRegion.get(), clientRegion.get(), RGN_OR));
         winrt::check_bool(SetWindowRgn(_interopWindowHandle, _dragBarRegion.get(), true));
     }
-
-    winrt::check_hresult(_UpdateFrameMargins());
 }
 
 // Method Description:
@@ -229,10 +277,13 @@ MARGINS NonClientIslandWindow::GetFrameMargins() const noexcept
 // - the HRESULT returned by DwmExtendFrameIntoClientArea.
 [[nodiscard]] HRESULT NonClientIslandWindow::_UpdateFrameMargins() const noexcept
 {
-    // Get the size of the borders we want to use. The sides and bottom will
-    // just be big enough for resizing, but the top will be as big as we need
-    // for the non-client content.
-    MARGINS margins = GetFrameMargins();
+    // Set frame margins with just a single pixel on the bottom. We don't
+    // really want a window frame at all - we're drawing all of it. We
+    // especially don't want a top margin - that's where the caption buttons
+    // are, and we're drawing those. So just set a single pixel on the bottom,
+    // because the method won't work with {0}.
+    MARGINS margins = { 0, 0, 0, 1 };
+
     // Extend the frame into the client area.
     return DwmExtendFrameIntoClientArea(_window.get(), &margins);
 }
@@ -386,29 +437,33 @@ RECT NonClientIslandWindow::GetMaxWindowRectInPixels(const RECT* const prcSugges
         }
         break;
     }
+
     case WM_EXITSIZEMOVE:
     {
         ForceResize();
         break;
     }
-    case WM_NCACTIVATE:
-    case WM_NCPAINT:
+
+    case WM_PAINT:
     {
         if (!_dragBar)
         {
             return 0;
         }
 
-        const auto hdc = wil::GetDC(_window.get());
+        PAINTSTRUCT ps{ 0 };
+        const auto hdc = wil::BeginPaint(_window.get(), &ps);
         if (hdc.get())
         {
             const auto scale = GetCurrentDpiScale();
             const auto dpi = ::GetDpiForWindow(_window.get());
+            // Get the dimensions of the drag borders for the sides of the window.
             const auto dragY = ::GetSystemMetricsForDpi(SM_CYDRAG, dpi);
             const auto dragX = ::GetSystemMetricsForDpi(SM_CXDRAG, dpi);
             const auto xPos = _isMaximized ? _maximizedMargins.cxLeftWidth : dragX;
             const auto yPos = _isMaximized ? _maximizedMargins.cyTopHeight : dragY;
 
+            // Create brush for borders, titlebar color.
             const auto backgroundBrush = _dragBar.Background();
             const auto backgroundSolidBrush = backgroundBrush.as<winrt::Windows::UI::Xaml::Media::SolidColorBrush>();
             const auto backgroundColor = backgroundSolidBrush.Color();
@@ -420,27 +475,34 @@ RECT NonClientIslandWindow::GetMaxWindowRectInPixels(const RECT* const prcSugges
             const auto cx = windowRect.right - windowRect.left;
             const auto cy = windowRect.bottom - windowRect.top;
 
+            // Fill in the _entire_ titlebar area.
+            RECT dragBarRect = {};
+            dragBarRect.left = xPos;
+            dragBarRect.right = xPos + cx;
+            dragBarRect.top = yPos;
+            dragBarRect.bottom = yPos + cy;
+            ::FillRect(hdc.get(), &dragBarRect, _backgroundBrush.get());
+
+            // Draw the top window border
             RECT clientRect = { 0, 0, cx, yPos };
             ::FillRect(hdc.get(), &clientRect, _backgroundBrush.get());
 
+            // Draw the left window border
             clientRect = { 0, 0, xPos, cy };
             ::FillRect(hdc.get(), &clientRect, _backgroundBrush.get());
 
+            // Draw the bottom window border
             clientRect = { 0, cy - yPos, cx, cy };
             ::FillRect(hdc.get(), &clientRect, _backgroundBrush.get());
 
+            // Draw the right window border
             clientRect = { cx - xPos, 0, cx, cy };
             ::FillRect(hdc.get(), &clientRect, _backgroundBrush.get());
-
-            RECT dragBarRect = GetDragAreaRect();
-            dragBarRect.left += xPos;
-            dragBarRect.right += xPos;
-            dragBarRect.bottom += yPos;
-            dragBarRect.top += yPos;
-            ::FillRect(hdc.get(), &dragBarRect, _backgroundBrush.get());
         }
+
         return 0;
     }
+
     case WM_LBUTTONDOWN:
     {
         POINT point1 = {};

src/cascadia/WindowsTerminal/NonClientIslandWindow.h

@@ -54,6 +54,7 @@ class NonClientIslandWindow : public IslandWindow
 
     void _HandleActivateWindow();
     bool _HandleWindowPosChanging(WINDOWPOS* const windowPos);
+    void _UpdateDragRegion();
 
     void OnDragBarSizeChanged(winrt::Windows::Foundation::IInspectable sender, winrt::Windows::UI::Xaml::SizeChangedEventArgs eventArgs);