I suppose this works for the titlebar in fomo

src/cascadia/WindowsTerminal/IslandWindow.cpp

@@ -842,7 +842,7 @@ void IslandWindow::SetTaskbarProgress(const size_t state, const size_t progress)
 }
 
 // From GdiEngine::s_SetWindowLongWHelper
-void _SetWindowLongWHelper(const HWND hWnd, const int nIndex, const LONG dwNewLong) noexcept
+void SetWindowLongWHelper(const HWND hWnd, const int nIndex, const LONG dwNewLong) noexcept
 {
     // SetWindowLong has strange error handling. On success, it returns the
     // previous Window Long value and doesn't modify the Last Error state. To
@@ -933,14 +933,14 @@ void IslandWindow::_SetIsBorderless(const bool borderlessEnabled)
 
     // First, modify regular window styles as appropriate
     auto windowStyle = _getDesiredWindowStyle();
-    _SetWindowLongWHelper(hWnd, GWL_STYLE, windowStyle);
+    SetWindowLongWHelper(hWnd, GWL_STYLE, windowStyle);
 
     // Now modify extended window styles as appropriate
     // When moving to fullscreen, remove the window edge style to avoid an
     // ugly border when not focused.
     auto exWindowStyle = GetWindowLongW(hWnd, GWL_EXSTYLE);
     WI_UpdateFlag(exWindowStyle, WS_EX_WINDOWEDGE, !_fullscreen);
-    _SetWindowLongWHelper(hWnd, GWL_EXSTYLE, exWindowStyle);
+    SetWindowLongWHelper(hWnd, GWL_EXSTYLE, exWindowStyle);
 
     // Resize the window, with SWP_FRAMECHANGED, to trigger user32 to
     // recalculate the non/client areas
@@ -1078,14 +1078,14 @@ void IslandWindow::_SetIsFullscreen(const bool fullscreenEnabled)
 
     // First, modify regular window styles as appropriate
     auto windowStyle = _getDesiredWindowStyle();
-    _SetWindowLongWHelper(hWnd, GWL_STYLE, windowStyle);
+    SetWindowLongWHelper(hWnd, GWL_STYLE, windowStyle);
 
     // Now modify extended window styles as appropriate
     // When moving to fullscreen, remove the window edge style to avoid an
     // ugly border when not focused.
     auto exWindowStyle = GetWindowLongW(hWnd, GWL_EXSTYLE);
     WI_UpdateFlag(exWindowStyle, WS_EX_WINDOWEDGE, !_fullscreen);
-    _SetWindowLongWHelper(hWnd, GWL_EXSTYLE, exWindowStyle);
+    SetWindowLongWHelper(hWnd, GWL_EXSTYLE, exWindowStyle);
 
     // Only change the window position if changing fullscreen state.
     if (fChangingFullscreen)

src/cascadia/WindowsTerminal/IslandWindow.h

@@ -6,6 +6,8 @@
 #include <winrt/TerminalApp.h>
 #include "../../cascadia/inc/cppwinrt_utils.h"
 
+void SetWindowLongWHelper(const HWND hWnd, const int nIndex, const LONG dwNewLong) noexcept;
+
 class IslandWindow :
     public BaseWindow<IslandWindow>
 {

src/cascadia/WindowsTerminal/NonClientIslandWindow.cpp

@@ -685,9 +685,13 @@ SIZE NonClientIslandWindow::GetTotalNonClientExclusiveSize(UINT dpi) const noexc
 // - the HRESULT returned by DwmExtendFrameIntoClientArea.
 void NonClientIslandWindow::_UpdateFrameMargins() const noexcept
 {
-    MARGINS margins = {};
+    MARGINS margins = { 0, 0, 0, 0 };
 
-    if (_GetTopBorderHeight() != 0)
+    if (_borderless)
+    {
+        margins.cyTopHeight = 1;
+    }
+    else if (_GetTopBorderHeight() != 0)
     {
         RECT frame = {};
         winrt::check_bool(::AdjustWindowRectExForDpi(&frame, GetWindowStyle(_window.get()), FALSE, 0, _currentDpi));
@@ -896,6 +900,16 @@ void NonClientIslandWindow::_SetIsBorderless(const bool borderlessEnabled)
         _titlebar.Visibility(_IsTitlebarVisible() ? Visibility::Visible : Visibility::Collapsed);
     }
 
+    // These are all useless.
+    // auto windowStyle = GetWindowLongW(GetHandle(), GWL_STYLE);
+    // WI_ClearAllFlags(windowStyle, WS_OVERLAPPEDWINDOW);
+    // WI_SetFlag(windowStyle, WS_SIZEBOX);
+    // // WI_SetFlag(windowStyle, WS_BORDER);
+    // WI_SetFlag(windowStyle, WS_POPUP);
+    // SetWindowLongWHelper(GetHandle(), GWL_STYLE, windowStyle);
+
+    _UpdateFrameMargins();
+
     // GH#4224 - When the auto-hide taskbar setting is enabled, then we don't
     // always get another window message to trigger us to remove the drag bar.
     // So, make sure to update the size of the drag region here, so that it