Account for the window frame when calculating initial position

src/cascadia/WindowsTerminal/AppHost.cpp

@@ -410,6 +410,7 @@ void AppHost::_HandleCreateWindow(const HWND hwnd, RECT proposedRect, LaunchMode
     // Get the size of a window we'd need to host that client rect. This will
     // add the titlebar space.
     const til::size nonClientSize = _window->GetTotalNonClientExclusiveSize(dpix);
+    const til::rectangle nonClientFrame = _window->GetNonClientFrame(dpix);
     adjustedWidth = islandWidth + nonClientSize.width<long>();
     adjustedHeight = islandHeight + nonClientSize.height<long>();
 
@@ -425,14 +426,18 @@ void AppHost::_HandleCreateWindow(const HWND hwnd, RECT proposedRect, LaunchMode
     const til::size desktopDimensions{ gsl::narrow<short>(nearestMonitorInfo.rcWork.right - nearestMonitorInfo.rcWork.left),
                                        gsl::narrow<short>(nearestMonitorInfo.rcWork.bottom - nearestMonitorInfo.rcWork.top) };
 
-    til::point origin{ (proposedRect.left),
+    // GH#10583 - Adjust the position of the rectangle to account for the size
+    // of the invisible borders on the left/right. We DON'T want to adjust this
+    // for the top here - the IslandWindow includes the titlebar in
+    // nonClientFrame.top, so adjusting for that would actually place the
+    // titlebar _off_ the monitor.
+    til::point origin{ (proposedRect.left + nonClientFrame.left<LONG>()),
                        (proposedRect.top) };
 
     if (_logic.IsQuakeWindow())
     {
         // If we just use rcWork by itself, we'll fail to account for the invisible
         // space reserved for the resize handles. So retrieve that size here.
-        const til::size ncSize{ _window->GetTotalNonClientExclusiveSize(dpix) };
         const til::size availableSpace = desktopDimensions + nonClientSize;
 
         origin = til::point{

src/cascadia/WindowsTerminal/IslandWindow.cpp

@@ -616,12 +616,20 @@ void IslandWindow::SetContent(winrt::Windows::UI::Xaml::UIElement content)
 }
 
 // Method Description:
-// - Gets the difference between window and client area size.
+// - Get the dimensions of our non-client area, as a rect where each component
+//   represents that side.
+// - The .left will be a negative number, to represent that the actual side of
+//   the non-client area is outside the border of our window. It's roughly 8px (
+//   * DPI scaling) to the left of the visible border.
+// - The .right component will be positive, indicating that the nonclient border
+//   is in the positive-x direction from the edge of our client area.
+// - This will also include our titlebar! It's in the nonclient area for us.
 // Arguments:
-// - dpi: dpi of a monitor on which the window is placed
-// Return Value
-// - The size difference
-SIZE IslandWindow::GetTotalNonClientExclusiveSize(const UINT dpi) const noexcept
+// - dpi: the scaling that we should use to calculate the border sizes.
+// Return Value:
+// - a RECT whose components represent the margins of the nonclient area,
+//   relative to the client area.
+RECT IslandWindow::GetNonClientFrame(const UINT dpi) const noexcept
 {
     const auto windowStyle = static_cast<DWORD>(GetWindowLong(_window.get(), GWL_STYLE));
     RECT islandFrame{};
@@ -630,7 +638,18 @@ SIZE IslandWindow::GetTotalNonClientExclusiveSize(const UINT dpi) const noexcept
     // 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_IF_WIN32_BOOL_FALSE(AdjustWindowRectExForDpi(&islandFrame, windowStyle, false, 0, dpi));
+    return islandFrame;
+}
 
+// Method Description:
+// - Gets the difference between window and client area size.
+// Arguments:
+// - dpi: dpi of a monitor on which the window is placed
+// Return Value
+// - The size difference
+SIZE IslandWindow::GetTotalNonClientExclusiveSize(const UINT dpi) const noexcept
+{
+    const auto islandFrame{ GetNonClientFrame(dpi) };
     return {
         islandFrame.right - islandFrame.left,
         islandFrame.bottom - islandFrame.top

src/cascadia/WindowsTerminal/IslandWindow.h

@@ -24,6 +24,7 @@ class IslandWindow :
     virtual void OnAppInitialized();
     virtual void SetContent(winrt::Windows::UI::Xaml::UIElement content);
     virtual void OnApplicationThemeChanged(const winrt::Windows::UI::Xaml::ElementTheme& requestedTheme);
+    virtual RECT GetNonClientFrame(const UINT dpi) const noexcept;
     virtual SIZE GetTotalNonClientExclusiveSize(const UINT dpi) const noexcept;
 
     virtual void Initialize();

src/cascadia/WindowsTerminal/NonClientIslandWindow.cpp

@@ -629,14 +629,21 @@ int NonClientIslandWindow::_GetResizeHandleHeight() const noexcept
 
     return DefWindowProc(GetHandle(), WM_SETCURSOR, wParam, lParam);
 }
-
 // Method Description:
-// - Gets the difference between window and client area size.
+// - Get the dimensions of our non-client area, as a rect where each component
+//   represents that side.
+// - The .left will be a negative number, to represent that the actual side of
+//   the non-client area is outside the border of our window. It's roughly 8px (
+//   * DPI scaling) to the left of the visible border.
+// - The .right component will be positive, indicating that the nonclient border
+//   is in the positive-x direction from the edge of our client area.
+// - This DOES NOT include our titlebar! It's in the client area for us.
 // Arguments:
-// - dpi: dpi of a monitor on which the window is placed
-// Return Value
-// - The size difference
-SIZE NonClientIslandWindow::GetTotalNonClientExclusiveSize(UINT dpi) const noexcept
+// - dpi: the scaling that we should use to calculate the border sizes.
+// Return Value:
+// - a RECT whose components represent the margins of the nonclient area,
+//   relative to the client area.
+RECT NonClientIslandWindow::GetNonClientFrame(UINT dpi) const noexcept
 {
     const auto windowStyle = static_cast<DWORD>(GetWindowLong(_window.get(), GWL_STYLE));
     RECT islandFrame{};
@@ -647,6 +654,18 @@ SIZE NonClientIslandWindow::GetTotalNonClientExclusiveSize(UINT dpi) const noexc
     LOG_IF_WIN32_BOOL_FALSE(AdjustWindowRectExForDpi(&islandFrame, windowStyle, false, 0, dpi));
 
     islandFrame.top = -topBorderVisibleHeight;
+    return islandFrame;
+}
+
+// Method Description:
+// - Gets the difference between window and client area size.
+// Arguments:
+// - dpi: dpi of a monitor on which the window is placed
+// Return Value
+// - The size difference
+SIZE NonClientIslandWindow::GetTotalNonClientExclusiveSize(UINT dpi) const noexcept
+{
+    const auto islandFrame{ GetNonClientFrame(dpi) };
 
     // If we have a titlebar, this is being called after we've initialized, and
     // we can just ask that titlebar how big it wants to be.

src/cascadia/WindowsTerminal/NonClientIslandWindow.h

@@ -37,6 +37,7 @@ class NonClientIslandWindow : public IslandWindow
 
     [[nodiscard]] virtual LRESULT MessageHandler(UINT const message, WPARAM const wparam, LPARAM const lparam) noexcept override;
 
+    virtual RECT GetNonClientFrame(UINT dpi) const noexcept override;
     virtual SIZE GetTotalNonClientExclusiveSize(UINT dpi) const noexcept override;
 
     void Initialize() override;