Add support for DECSCNM in Windows Terminal (#6809)

## Summary of the Pull Request

This PR adds full support for the `DECSCNM` reverse screen mode in the Windows Terminal to align with the implementation in conhost.

## References

* The conhost implementation of `DECSCNM` was in PR #3817.
* WT originally inherited that functionality via the colors being passed through, but that behaviour was lost in PR #6506.

## PR Checklist
* [x] Closes #6622
* [x] CLA signed.
* [ ] Tests added/passed
* [ ] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
* [ ] Schema updated.
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #6622

## Detailed Description of the Pull Request / Additional comments

The `AdaptDispatch::SetScreenMode` now checks if it's in conpty mode and simply returns false to force a pass-through of the mode change. And the `TerminalDispatch` now has its own `SetScreenMode` implementation that tracks any changes to the reversed state, and triggers a redraw in the renderer.

To make the renderer work, we just needed to update the `GetForegroundColor` and `GetBackgroundColor` methods of the terminal's `IRenderData` implementation to check the reversed state, and switch the colors being calculated, the same way the `LookupForegroundColor` and `LookupBackgroundColor` methods work in the conhost `Settings` class.

## Validation Steps Performed

I've manually tested the `DECSCNM` functionality for Windows Terminal in Vttest, and also with some of my own test scripts.

src/cascadia/TerminalCore/ITerminalApi.hpp

@@ -46,6 +46,7 @@ namespace Microsoft::Terminal::Core
         virtual bool EnableWin32InputMode(const bool win32InputMode) noexcept = 0;
         virtual bool SetCursorKeysMode(const bool applicationMode) noexcept = 0;
         virtual bool SetKeypadMode(const bool applicationMode) noexcept = 0;
+        virtual bool SetScreenMode(const bool reverseMode) noexcept = 0;
         virtual bool EnableVT200MouseMode(const bool enabled) noexcept = 0;
         virtual bool EnableUTF8ExtendedMouseMode(const bool enabled) noexcept = 0;
         virtual bool EnableSGRExtendedMouseMode(const bool enabled) noexcept = 0;

src/cascadia/TerminalCore/Terminal.cpp

@@ -41,6 +41,7 @@ Terminal::Terminal() :
     _colorTable{},
     _defaultFg{ RGB(255, 255, 255) },
     _defaultBg{ ARGB(0, 0, 0, 0) },
+    _screenReversed{ false },
     _pfnWriteInput{ nullptr },
     _scrollOffset{ 0 },
     _snapOnInput{ true },

src/cascadia/TerminalCore/Terminal.hpp

@@ -99,6 +99,7 @@ public:
     bool EnableWin32InputMode(const bool win32InputMode) noexcept override;
     bool SetCursorKeysMode(const bool applicationMode) noexcept override;
     bool SetKeypadMode(const bool applicationMode) noexcept override;
+    bool SetScreenMode(const bool reverseMode) noexcept override;
     bool EnableVT200MouseMode(const bool enabled) noexcept override;
     bool EnableUTF8ExtendedMouseMode(const bool enabled) noexcept override;
     bool EnableSGRExtendedMouseMode(const bool enabled) noexcept override;
@@ -208,6 +209,7 @@ private:
     std::array<COLORREF, XTERM_COLOR_TABLE_SIZE> _colorTable;
     COLORREF _defaultFg;
     COLORREF _defaultBg;
+    bool _screenReversed;
 
     bool _snapOnInput;
     bool _altGrAliasing;

src/cascadia/TerminalCore/TerminalApi.cpp

@@ -471,6 +471,17 @@ bool Terminal::SetKeypadMode(const bool applicationMode) noexcept
     return true;
 }
 
+bool Terminal::SetScreenMode(const bool reverseMode) noexcept
+try
+{
+    _screenReversed = reverseMode;
+
+    // Repaint everything - the colors will have changed
+    _buffer->GetRenderTarget().TriggerRedrawAll();
+    return true;
+}
+CATCH_LOG_RETURN_FALSE()
+
 bool Terminal::EnableVT200MouseMode(const bool enabled) noexcept
 {
     _terminalInput->EnableDefaultTracking(enabled);

src/cascadia/TerminalCore/TerminalDispatch.cpp

@@ -253,6 +253,18 @@ bool TerminalDispatch::SetCursorKeysMode(const bool applicationMode) noexcept
     return true;
 }
 
+// Routine Description:
+// - DECSCNM - Sets the screen mode to either normal or reverse.
+//    When in reverse screen mode, the background and foreground colors are switched.
+// Arguments:
+// - reverseMode - set to true to enable reverse screen mode, false for normal mode.
+// Return Value:
+// - True if handled successfully. False otherwise.
+bool TerminalDispatch::SetScreenMode(const bool reverseMode) noexcept
+{
+    return _terminalApi.SetScreenMode(reverseMode);
+}
+
 // Method Description:
 // - win32-input-mode: Enable sending full input records encoded as a string of
 //   characters to the client application.
@@ -390,6 +402,9 @@ bool TerminalDispatch::_PrivateModeParamsHelper(const DispatchTypes::PrivateMode
         // set - Enable Application Mode, reset - Normal mode
         success = SetCursorKeysMode(enable);
         break;
+    case DispatchTypes::PrivateModeParams::DECSCNM_ScreenMode:
+        success = SetScreenMode(enable);
+        break;
     case DispatchTypes::PrivateModeParams::VT200_MOUSE_MODE:
         success = EnableVT200MouseMode(enable);
         break;
@@ -493,8 +508,8 @@ bool TerminalDispatch::HardReset() noexcept
     success = EraseInDisplay(DispatchTypes::EraseType::All) && success;
     success = EraseInDisplay(DispatchTypes::EraseType::Scrollback) && success;
 
-    // // Set the DECSCNM screen mode back to normal.
-    // success = SetScreenMode(false) && success;
+    // Set the DECSCNM screen mode back to normal.
+    success = SetScreenMode(false) && success;
 
     // Cursor to 1,1 - the Soft Reset guarantees this is absolute
     success = CursorPosition(1, 1) && success;

src/cascadia/TerminalCore/TerminalDispatch.hpp

@@ -47,6 +47,7 @@ public:
 
     bool SetCursorKeysMode(const bool applicationMode) noexcept override; // DECCKM
     bool SetKeypadMode(const bool applicationMode) noexcept override; // DECKPAM, DECKPNM
+    bool SetScreenMode(const bool reverseMode) noexcept override; // DECSCNM
 
     bool SoftReset() noexcept override; // DECSTR
     bool HardReset() noexcept override; // RIS

src/cascadia/TerminalCore/terminalrenderdata.cpp

@@ -52,15 +52,32 @@ const TextAttribute Terminal::GetDefaultBrushColors() noexcept
 
 const COLORREF Terminal::GetForegroundColor(const TextAttribute& attr) const noexcept
 {
-    return 0xff000000 | attr.CalculateRgbForeground({ _colorTable.data(), _colorTable.size() }, _defaultFg, _defaultBg);
+    COLORREF fgColor{};
+    if (_screenReversed)
+    {
+        fgColor = attr.CalculateRgbBackground({ _colorTable.data(), _colorTable.size() }, _defaultFg, _defaultBg);
+    }
+    else
+    {
+        fgColor = attr.CalculateRgbForeground({ _colorTable.data(), _colorTable.size() }, _defaultFg, _defaultBg);
+    }
+    return 0xff000000 | fgColor;
 }
 
 const COLORREF Terminal::GetBackgroundColor(const TextAttribute& attr) const noexcept
 {
-    const auto bgColor = attr.CalculateRgbBackground({ _colorTable.data(), _colorTable.size() }, _defaultFg, _defaultBg);
+    COLORREF bgColor{};
+    if (_screenReversed)
+    {
+        bgColor = attr.CalculateRgbForeground({ _colorTable.data(), _colorTable.size() }, _defaultFg, _defaultBg);
+    }
+    else
+    {
+        bgColor = attr.CalculateRgbBackground({ _colorTable.data(), _colorTable.size() }, _defaultFg, _defaultBg);
+    }
     // We only care about alpha for the default BG (which enables acrylic)
     // If the bg isn't the default bg color, or reverse video is enabled, make it fully opaque.
-    if (!attr.BackgroundIsDefault() || attr.IsReverseVideo())
+    if (!attr.BackgroundIsDefault() || (attr.IsReverseVideo() ^ _screenReversed))
     {
         return 0xff000000 | bgColor;
     }
@@ -213,10 +230,9 @@ void Terminal::UnlockConsole() noexcept
 
 // Method Description:
 // - Returns whether the screen is inverted;
-//   This state is not currently known to Terminal.
 // Return Value:
 // - false.
 bool Terminal::IsScreenReversed() const noexcept
 {
-    return false;
+    return _screenReversed;
 }

src/terminal/adapter/adaptDispatch.cpp

@@ -1205,6 +1205,12 @@ bool AdaptDispatch::SetAnsiMode(const bool ansiMode)
 // - True if handled successfully. False otherwise.
 bool AdaptDispatch::SetScreenMode(const bool reverseMode)
 {
+    // If we're a conpty, always return false
+    if (_pConApi->IsConsolePty())
+    {
+        return false;
+    }
+
     return _pConApi->PrivateSetScreenMode(reverseMode);
 }