Fix the DECTCEM reset position in the conpty stream

src/renderer/vt/XtermEngine.cpp

@@ -45,6 +45,7 @@ XtermEngine::XtermEngine(_In_ wil::unique_hfile hPipe,
     // visible, then PaintCursor will be called, and we'll set this to true
     // during the frame.
     _nextCursorIsVisible = false;
+    _startOfFrameBufferIndex = _buffer.size();
 
     // Do not perform synchronization clearing in passthrough mode.
     // In passthrough, the terminal leads and we follow what it is
@@ -112,7 +113,7 @@ XtermEngine::XtermEngine(_In_ wil::unique_hfile hPipe,
         // by prepending a cursor off.
         if (_lastCursorIsVisible != Tribool::False)
         {
-            _buffer.insert(0, "\x1b[?25l");
+            _buffer.insert(_startOfFrameBufferIndex, "\x1b[?25l");
             _lastCursorIsVisible = Tribool::False;
         }
         // If the cursor was NOT previously visible, then that's fine! we don't

src/renderer/vt/state.cpp

@@ -158,6 +158,7 @@ void VtEngine::_flushImpl() noexcept
     {
         const auto fSuccess = WriteFile(_hFile.get(), _buffer.data(), gsl::narrow_cast<DWORD>(_buffer.size()), nullptr, nullptr);
         _buffer.clear();
+        _startOfFrameBufferIndex = 0;
         if (!fSuccess)
         {
             LOG_LAST_ERROR();

src/renderer/vt/vtrenderer.hpp

@@ -93,6 +93,7 @@ namespace Microsoft::Console::Render
     protected:
         wil::unique_hfile _hFile;
         std::string _buffer;
+        size_t _startOfFrameBufferIndex = 0;
 
         std::string _formatBuffer;
         std::string _conversionBuffer;