Fix clear buffer command (#17884)

Without a VT "renderer" there's no implicit output anymore when
calling `ClearPseudoConsole`. The fix is trivial, but it works
slightly different from before: Previously, we would preserve
the line the cursor is on, while this PR doesn't do that.
I felt like there's not much merit in preserving the line,
because it may be a multi-line prompt which won't work with that.

Closes #17867

## Validation Steps Performed
Bind 3 different actions to the 3 variants of "Clear buffer"
and test them. They work. ✅

src/cascadia/TerminalControl/ControlCore.cpp

@@ -2208,19 +2208,32 @@ namespace winrt::Microsoft::Terminal::Control::implementation
     // - <none>
     void ControlCore::ClearBuffer(Control::ClearBufferType clearType)
     {
-        if (clearType == Control::ClearBufferType::Scrollback || clearType == Control::ClearBufferType::All)
+        std::wstring_view command;
+        switch (clearType)
+        {
+        case ClearBufferType::Screen:
+            command = L"\x1b[H\x1b[2J";
+            break;
+        case ClearBufferType::Scrollback:
+            command = L"\x1b[3J";
+            break;
+        case ClearBufferType::All:
+            command = L"\x1b[H\x1b[2J\x1b[3J";
+            break;
+        }
+
         {
             const auto lock = _terminal->LockForWriting();
-            _terminal->EraseScrollback();
+            _terminal->Write(command);
         }
 
         if (clearType == Control::ClearBufferType::Screen || clearType == Control::ClearBufferType::All)
         {
-            // Send a signal to conpty to clear the buffer.
             if (auto conpty{ _connection.try_as<TerminalConnection::ConptyConnection>() })
             {
-                // ConPTY will emit sequences to sync up our buffer with its new
-                // contents.
+                // Since the clearing of ConPTY occurs asynchronously, this call can result weird issues,
+                // where a console application still sees contents that we've already deleted, etc.
+                // The correct way would be for ConPTY to emit the appropriate CSI n J sequences.
                 conpty.ClearBuffer();
             }
         }

src/cascadia/TerminalCore/Terminal.cpp

@@ -212,12 +212,6 @@ void Terminal::SetCursorStyle(const DispatchTypes::CursorStyle cursorStyle)
     engine.Dispatch().SetCursorStyle(cursorStyle);
 }
 
-void Terminal::EraseScrollback()
-{
-    auto& engine = reinterpret_cast<OutputStateMachineEngine&>(_stateMachine->Engine());
-    engine.Dispatch().EraseInDisplay(DispatchTypes::EraseType::Scrollback);
-}
-
 bool Terminal::IsXtermBracketedPasteModeEnabled() const noexcept
 {
     return _systemMode.test(Mode::BracketedPaste);

src/cascadia/TerminalCore/Terminal.hpp

@@ -94,7 +94,6 @@ public:
     void UpdateAppearance(const winrt::Microsoft::Terminal::Core::ICoreAppearance& appearance);
     void SetFontInfo(const FontInfo& fontInfo);
     void SetCursorStyle(const ::Microsoft::Console::VirtualTerminal::DispatchTypes::CursorStyle cursorStyle);
-    void EraseScrollback();
     bool IsXtermBracketedPasteModeEnabled() const noexcept;
     std::wstring_view GetWorkingDirectory() noexcept;
 

src/cascadia/UnitTests_Control/ControlCoreTests.cpp

@@ -304,9 +304,9 @@ namespace ControlUnitTests
 
         Log::Comment(L"Check the buffer after the clear");
         VERIFY_ARE_EQUAL(20, core->_terminal->GetViewport().Height());
-        VERIFY_ARE_EQUAL(21, core->ScrollOffset());
+        VERIFY_ARE_EQUAL(41, core->ScrollOffset());
         VERIFY_ARE_EQUAL(20, core->ViewHeight());
-        VERIFY_ARE_EQUAL(41, core->BufferHeight());
+        VERIFY_ARE_EQUAL(61, core->BufferHeight());
 
         // In this test, we can't actually check if we cleared the buffer
         // contents. ConPTY will handle the actual clearing of the buffer

src/host/PtySignalInputThread.cpp

@@ -7,6 +7,7 @@
 
 #include "output.h"
 #include "handle.h"
+#include "_stream.h"
 #include "../interactivity/inc/ServiceLocator.hpp"
 
 using namespace Microsoft::Console;
@@ -197,7 +198,9 @@ void PtySignalInputThread::_DoClearBuffer() const
     }
 
     auto& gci = ServiceLocator::LocateGlobals().getConsoleInformation();
-    THROW_IF_FAILED(gci.GetActiveOutputBuffer().ClearBuffer());
+    auto& screenInfo = gci.GetActiveOutputBuffer();
+    auto& stateMachine = screenInfo.GetStateMachine();
+    stateMachine.ProcessString(L"\x1b[H\x1b[2J");
 }
 
 void PtySignalInputThread::_DoShowHide(const ShowHideData& data)

src/host/screenInfo.cpp

@@ -2129,38 +2129,6 @@ void SCREEN_INFORMATION::SetViewport(const Viewport& newViewport,
     Tracing::s_TraceWindowViewport(_viewport);
 }
 
-// Method Description:
-// - Clear the entire contents of the viewport, except for the cursor's row,
-//   which is moved to the top line of the viewport.
-// - This is used exclusively by ConPTY to support GH#1193, GH#1882. This allows
-//   a terminal to clear the contents of the ConPTY buffer, which is important
-//   if the user would like to be able to clear the terminal-side buffer.
-// Arguments:
-// - <none>
-// Return Value:
-// - S_OK
-[[nodiscard]] HRESULT SCREEN_INFORMATION::ClearBuffer()
-{
-    // Rotate the buffer to bring the cursor row to the top of the viewport.
-    const auto cursorPos = _textBuffer->GetCursor().GetPosition();
-    for (auto i = 0; i < cursorPos.y; i++)
-    {
-        _textBuffer->IncrementCircularBuffer();
-    }
-
-    // Erase everything below that point.
-    RETURN_IF_FAILED(SetCursorPosition({ 0, 1 }, false));
-    auto& engine = reinterpret_cast<OutputStateMachineEngine&>(_stateMachine->Engine());
-    engine.Dispatch().EraseInDisplay(DispatchTypes::EraseType::ToEnd);
-
-    // Restore the original cursor x offset, but now on the first row.
-    RETURN_IF_FAILED(SetCursorPosition({ cursorPos.x, 0 }, false));
-
-    _textBuffer->TriggerRedrawAll();
-
-    return S_OK;
-}
-
 // Routine Description:
 // - Writes cells to the output buffer at the cursor position.
 // Arguments:

src/host/screenInfo.hpp

@@ -203,8 +203,6 @@ public:
     void SetDefaultAttributes(const TextAttribute& attributes,
                               const TextAttribute& popupAttributes);
 
-    [[nodiscard]] HRESULT ClearBuffer();
-
     void UpdateBottom();
 
     FontInfo& GetCurrentFont() noexcept;