Fix ClearScreen to properly scroll

The previous change scrolled the wrong test,
losing the bottom instead of the top of the buffer.

Author: lzybkr

PSReadLine/Changes.txt

@@ -13,7 +13,7 @@ New features:
 Bug fixes:
 * Occasional exception with AcceptAndGetNext (Ctrl+O) followed by something other than Enter
 * Event handlers that register for the engine event PowerShell.OnIdle should work now.
-* ClearScreen now just clears the visible screen, not the entire buffer.
+* ClearScreen now scrolls the screen to preserve as much output as possible
 * Fix exception on error input during rendering after certain keywords like process, begin, or end.
 * Fix exception after undo from menu completion
 * Support CancelLine (Ctrl+C) and Abort (Ctrl+G in emacs) to cancel DigitArgument

PSReadLine/Movement.cs

@@ -413,10 +413,17 @@ public static void GotoBrace(ConsoleKeyInfo? key = null, object arg = null)
         /// </summary>
         public static void ClearScreen(ConsoleKeyInfo? key = null, object arg = null)
         {
-            int top = Console.WindowTop;
-            WriteBlankLines(Console.WindowHeight, top);
-            _singleton._initialY = top;
-            _singleton.Render();
+            if (_singleton._initialY + Console.WindowHeight > Console.BufferHeight)
+            {
+                var scrollCount = _singleton._initialY - Console.WindowTop;
+                ScrollBuffer(scrollCount);
+                _singleton._initialY -= scrollCount;
+                Console.SetCursorPosition(Console.CursorLeft, Console.CursorTop - scrollCount);
+            }
+            else
+            {
+                Console.SetWindowPosition(0, _singleton._initialY);
+            }
         }
 
         // Try to convert the arg to a char, return 0 for failure