Fix ReverseStringBuilder bug

Author: MackinnonBuck

src/Components/Forms/test/ReverseStringBuilderTest.cs

@@ -92,6 +92,24 @@ public void ToString_Works_AfterExceedingStackAllocatedBuffer()
         Assert.Equal(1, builder.SequenceSegmentCount);
     }
 
+    [Fact]
+    public void ToString_Works_AfterExceedingStackAllocatedBuffer_WithUnusedSpaceInInitialBuffer()
+    {
+        const int InitialBufferSize = 64;
+        var builder = new ReverseStringBuilder(stackalloc char[InitialBufferSize]);
+
+        var firstChunk = new string('A', InitialBufferSize - 10);
+        var secondChunk = new string('B', 20);
+
+        builder.InsertFront(firstChunk);
+        builder.InsertFront(secondChunk);
+
+        var expected = secondChunk + firstChunk;
+        var actual = builder.ToString();
+
+        Assert.Equal(expected, actual);
+    }
+
     [Fact]
     public void ToString_Works_AfterExpandingIntoMultipleBuffersFromEstimatedStringSize()
     {

src/Components/Shared/src/ExpressionFormatting/ReverseStringBuilder.cs

@@ -17,7 +17,7 @@ internal ref struct ReverseStringBuilder
     private SequenceSegment? _fallbackSequenceSegment;
 
     // For testing.
-    internal int SequenceSegmentCount => _fallbackSequenceSegment?.Count() ?? 0;
+    internal readonly int SequenceSegmentCount => _fallbackSequenceSegment?.Count() ?? 0;
 
     public ReverseStringBuilder(int conservativeEstimatedStringLength)
     {
@@ -33,7 +33,7 @@ public ReverseStringBuilder(Span<char> initialBuffer)
         _nextEndIndex = _currentBuffer.Length;
     }
 
-    public bool Empty => _nextEndIndex == _currentBuffer.Length;
+    public readonly bool Empty => _nextEndIndex == _currentBuffer.Length;
 
     public void InsertFront(scoped ReadOnlySpan<char> span)
     {
@@ -60,13 +60,13 @@ public void InsertFront(scoped ReadOnlySpan<char> span)
             var newBuffer = s_arrayPool.Rent(sizeToRent);
             _fallbackSequenceSegment = new(newBuffer);
 
-            _nextEndIndex = newBuffer.Length - _currentBuffer.Length;
-            _currentBuffer.CopyTo(newBuffer.AsSpan()[_nextEndIndex..]);
-            _currentBuffer = newBuffer;
+            var newEndIndex = newBuffer.Length - _currentBuffer.Length + _nextEndIndex;
+            _currentBuffer[_nextEndIndex..].CopyTo(newBuffer.AsSpan(newEndIndex));
+            newEndIndex -= span.Length;
+            span.CopyTo(newBuffer.AsSpan(newEndIndex));
 
-            startIndex = _nextEndIndex - span.Length;
-            span.CopyTo(_currentBuffer[startIndex..]);
-            _nextEndIndex = startIndex;
+            _currentBuffer = newBuffer;
+            _nextEndIndex = newEndIndex;
         }
         else
         {