Properly fix ConPTY buffer corking

src/renderer/vt/invalidate.cpp

@@ -112,6 +112,10 @@ CATCH_RETURN();
     //      end paint to specifically handle this.
     _circled = circled;
 
+    // If we flushed for any reason other than circling (i.e, a sequence that we
+    // didn't understand), we don't need to push the buffer out on EndPaint.
+    _noFlushOnEnd = !circled;
+
     _trace.TraceTriggerCircling(*pForcePaint);
     return S_OK;
 }

src/renderer/vt/paint.cpp

@@ -94,7 +94,19 @@ using namespace Microsoft::Console::Types;
         RETURN_IF_FAILED(_MoveCursor(_deferredCursorPos));
     }
 
-    _Flush();
+    // If this frame was triggered because we encountered a VT sequence which
+    // required the buffered state to get printed, we don't want to flush this
+    // frame to the pipe. That might result in us rendering half the output of a
+    // particular frame (as emitted by the client).
+    //
+    // Instead, we'll leave this frame in _buffer, and just keep appending to
+    // it as needed.
+    if (!_noFlushOnEnd)
+    {
+        _Flush();
+    }
+
+    _noFlushOnEnd = false;
     return S_OK;
 }
 

src/renderer/vt/state.cpp

@@ -136,10 +136,19 @@ CATCH_RETURN();
 
 void VtEngine::_Flush() noexcept
 {
-    if (!_corked && !_buffer.empty())
+    if (_buffer.empty())
+    {
+        return;
+    }
+
+    if (!_corked)
     {
         _flushImpl();
+        return;
     }
+
+    // Defer the flush until someone calls Cork(false).
+    _flushRequested = true;
 }
 
 // _corked is often true and separating _flushImpl() out allows _flush() to be inlined.
@@ -167,7 +176,13 @@ void VtEngine::_flushImpl() noexcept
 void VtEngine::Cork(bool corked) noexcept
 {
     _corked = corked;
-    _Flush();
+
+    // Now do the deferred flush from a previous call to _Flush().
+    if (!corked && _flushRequested)
+    {
+        _flushRequested = false;
+        _flushImpl();
+    }
 }
 
 // Method Description:

src/renderer/vt/vtrenderer.hpp

@@ -138,7 +138,9 @@ namespace Microsoft::Console::Render
 
         bool _resizeQuirk{ false };
         bool _passthrough{ false };
+        bool _noFlushOnEnd{ false };
         bool _corked{ false };
+        bool _flushRequested{ false };
         std::optional<TextColor> _newBottomLineBG{ std::nullopt };
 
         [[nodiscard]] HRESULT _WriteFill(const size_t n, const char c) noexcept;