Logic and UX improvements to screen reader renderer

Author: andyleejordan

PSReadLine/Render.cs

@@ -310,40 +310,51 @@ static int FindCommonPrefixLength(string leftStr, string rightStr)
             // Make cursor invisible while we're rendering.
             _console.CursorVisible = false;
 
-            // Calculate what to render and where to start the rendering.
-            // TODO: Short circuit optimization when currentBuffer == previousBuffer.
-            int commonPrefixLength = FindCommonPrefixLength(previousBuffer, currentBuffer);
-
-            if (commonPrefixLength > 0 && commonPrefixLength == previousBuffer.Length)
+            if (currentBuffer == previousBuffer)
             {
-                // Previous buffer is a complete prefix of current buffer.
-                // Just append the new data.
-                var appendedData = currentBuffer.Substring(commonPrefixLength);
-                _console.Write(appendedData);
+                // No-op, such as when selecting text or otherwise re-entering.
             }
-            else if (commonPrefixLength > 0)
-            {
-                // Buffers share a common prefix but previous buffer has additional content.
-                // Move cursor to where the difference starts, clear forward, and write the data.
-                var diffPoint = ConvertOffsetToPoint(commonPrefixLength, buffer);
-                _console.SetCursorPosition(diffPoint.X, diffPoint.Y);
-                var changedData = currentBuffer.Substring(commonPrefixLength);
-                _console.Write("\x1b[0J");
-                _console.Write(changedData);
-            }
-            else
+            else if (previousBuffer.Length == 0)
             {
-                // No common prefix, rewrite entire buffer.
+                // Previous buffer was empty so we just render the current buffer.
                 _console.SetCursorPosition(_initialX, _initialY);
-                _console.Write("\x1b[0J");
                 _console.Write(currentBuffer);
             }
+            else
+            {
+                // Calculate what to render and where to start the rendering.
+                int commonPrefixLength = FindCommonPrefixLength(previousBuffer, currentBuffer);
 
+                // If we're scrolling through history we always want to re-render.
+                // Writing only the diff in this scenario is a weird UX.
+                if (commonPrefixLength > 0 && _anyHistoryCommandCount == 0)
+                {
+                    // We need to differentially render, possibly with a partial rewrite.
+                    if (commonPrefixLength != previousBuffer.Length)
+                    {
+                        // The buffers share a common prefix but the previous buffer has additional content.
+                        // Move cursor to where the difference starts and clear so we can rewrite.
+                        var diffPoint = ConvertOffsetToPoint(commonPrefixLength, buffer);
+                        _console.SetCursorPosition(diffPoint.X, diffPoint.Y);
+                        _console.Write("\x1b[0J");
+                    } // Otherwise the previous buffer is a complete prefix and we just write.
+
+                    var diffData = currentBuffer.Substring(commonPrefixLength);
+                    _console.Write(diffData);
+                }
+                else
+                {
+                    // The buffers are completely different so we need to rewrite from the start.
+                    _console.SetCursorPosition(_initialX, _initialY);
+                    _console.Write("\x1b[0J");
+                    _console.Write(currentBuffer);
+                }
+            }
+            
             // If we had to wrap to render everything, update _initialY
             var endPoint = ConvertOffsetToPoint(currentBuffer.Length, buffer);
             if (endPoint.Y >= bufferHeight)
             {
-
                 // We had to scroll to render everything, update _initialY.
                 int offset = 1; // Base case to handle zero-indexing.
                 if (endPoint.X == 0)