Fix correct way to get substring based on buffercells when encountering CJK chars (#1100)

Author: SteveL-MSFT

PSReadLine/Completion.cs

@@ -805,7 +805,7 @@ private static string GetMenuItem(string item, int columnWidth)
             }
             else if (spacesNeeded < 0)
             {
-                item = item.Substring(0, columnWidth - 3) + "...";
+                item = SubstringByCells(item, columnWidth - 3) + "...";
             }
 
             return item;

PSReadLine/Render.cs

@@ -371,7 +371,7 @@ private bool RenderErrorPrompt(RenderData renderData, string defaultColor)
 
             if (string.IsNullOrEmpty(promptText) || _initialY < 0)
             {
-                // No need to flip the prompt color if either the error prompt is not defined 
+                // No need to flip the prompt color if either the error prompt is not defined
                 // or the initial cursor point has already been scrolled off the buffer.
                 return false;
             }
@@ -875,6 +875,25 @@ private static int LengthInBufferCells(char c)
             return 1 + (isWide ? 1 : 0);
         }
 
+        private static string SubstringByCells(string text, int countOfCells)
+        {
+            int length = 0;
+            int index = 0;
+
+            foreach (char c in text)
+            {
+                if (length >= countOfCells)
+                {
+                    return text.Substring(0, index);
+                }
+
+                length += LengthInBufferCells(c);
+                index++;
+            }
+
+            return string.Empty;
+        }
+
         private string GetTokenColor(Token token)
         {
             if ((token.TokenFlags & TokenFlags.CommandName) != 0)