Fix cooked read reflow under ConPTY (#17668)

This delays the CSI J until we know the new origin of the prompt.
That way it's at the right (reflowed) position.

## Validation Steps Performed
* conhost
  * Print a ton of text
  * Write a prompt of a hundred chars
  * Resize the window very narrow / wide
  * Works ✅
* Windows Terminal
  * Write a prompt of a hundred chars
  * Resize the window very narrow / wide
  * Works ✅

Author: lhecker

src/host/readDataCooked.cpp

@@ -295,10 +295,14 @@ void COOKED_READ_DATA::EraseBeforeResize()
 
     _redrawPending = true;
 
-    std::wstring output;
-    _appendCUP(output, _originInViewport);
-    output.append(L"\x1b[J");
-    WriteCharsVT(_screenInfo, output);
+    // Position the cursor the start of the prompt before reflow.
+    // Then, after reflow, we'll be able to ask the buffer where it went (the new origin).
+    // This uses the buffer APIs directly, so that we don't emit unnecessary VT into ConPTY's output.
+    auto& textBuffer = _screenInfo.GetTextBuffer();
+    auto& cursor = textBuffer.GetCursor();
+    auto cursorPos = _originInViewport;
+    _screenInfo.GetVtPageArea().ConvertFromOrigin(&cursorPos);
+    cursor.SetPosition(cursorPos);
 }
 
 // The counter-part to EraseBeforeResize().
@@ -322,6 +326,10 @@ void COOKED_READ_DATA::RedrawAfterResize()
     _bufferDirtyBeg = 0;
     _dirty = !_buffer.empty();
 
+    // Let _redisplay() know to inject a CSI J at the start of the output.
+    // This ensures we fully erase the previous contents, that are now in disarray.
+    _clearPending = true;
+
     _redisplay();
 }
 
@@ -1081,6 +1089,13 @@ void COOKED_READ_DATA::_redisplay()
 
     std::wstring output;
 
+    if (_clearPending)
+    {
+        _clearPending = false;
+        _appendCUP(output, _originInViewport);
+        output.append(L"\x1b[J");
+    }
+
     // Disable the cursor when opening a popup, reenable it when closing them.
     if (const auto popupOpened = !_popups.empty(); _popupOpened != popupOpened)
     {

src/host/readDataCooked.hpp

@@ -164,6 +164,7 @@ class COOKED_READ_DATA final : public ReadData
     bool _insertMode = false;
     bool _dirty = false;
     bool _redrawPending = false;
+    bool _clearPending = false;
 
     til::point _originInViewport;
     // This value is in the pager coordinate space. (0,0) is the first character of the