Fix doubled index on macro opcodes in the IL view

The IL view renders a macro opcode like ldarg.0 as the mnemonic (an opcode
reference) followed by the index written as a separate clickable local or
parameter reference. AvaloniaEditTextOutput's omitSuffix path used
TrimEnd('.') to drop the index, but the name ends in the index digit, not a
dot, so it was a no-op: the full "ldarg.0" was written and the index appended
again, producing "ldarg.00" (and likewise ldloc.NN / stloc.NN).

PlainTextOutput and the WPF AvalonEditTextOutput strip everything after the
last dot; the Avalonia port regressed this. Restore that behavior. Only the UI
was affected -- ilspycmd and the disassembler tests use PlainTextOutput.

Assisted-by: Claude:claude-opus-4-8:Claude Code

ILSpy.Tests/TextView/OpCodeReferenceTests.cs

@@ -0,0 +1,74 @@
+// Copyright (c) 2026 AlphaSierraPapa for the SharpDevelop Team
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System.Reflection.Metadata;
+
+using AwesomeAssertions;
+
+using ICSharpCode.Decompiler.Disassembler;
+using ICSharpCode.Decompiler.Metadata;
+using ICSharpCode.ILSpy.TextView;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests.TextView;
+
+/// <summary>
+/// The IL view renders a macro opcode such as "ldarg.0" by writing the mnemonic as an opcode
+/// reference with the index omitted, then writing the index as a separate clickable local or
+/// parameter reference. The omitSuffix path must keep the mnemonic up to the dot ("ldarg.") so the
+/// index is written exactly once; otherwise the opcode reads "ldarg.00".
+/// </summary>
+[TestFixture]
+public class OpCodeReferenceTests
+{
+	static OpCodeInfo MacroOpCode(ILOpCode opCode) => new(opCode, opCode.GetDisplayName());
+
+	[Test]
+	public void WriteReference_OmitSuffix_Keeps_The_Mnemonic_Up_To_The_Dot()
+	{
+		var output = new AvaloniaEditTextOutput();
+
+		output.WriteReference(MacroOpCode(ILOpCode.Ldarg_0), omitSuffix: true);
+
+		output.GetText().Should().Be("ldarg.");
+	}
+
+	[Test]
+	public void WriteReference_Without_OmitSuffix_Writes_The_Full_Opcode()
+	{
+		var output = new AvaloniaEditTextOutput();
+
+		output.WriteReference(MacroOpCode(ILOpCode.Ldarg_0));
+
+		output.GetText().Should().Be("ldarg.0");
+	}
+
+	[Test]
+	public void Macro_Opcode_Renders_Its_Index_Exactly_Once()
+	{
+		// Mirrors MethodBodyDisassembler.WriteOpCode for a macro opcode: the mnemonic (index omitted)
+		// followed by the index as a local/parameter reference.
+		var output = new AvaloniaEditTextOutput();
+
+		output.WriteReference(MacroOpCode(ILOpCode.Ldarg_0), omitSuffix: true);
+		output.WriteLocalReference("0", "param_0");
+
+		output.GetText().Should().Be("ldarg.0");
+	}
+}

ILSpy/TextView/AvaloniaEditTextOutput.cs

@@ -180,7 +180,13 @@ namespace ICSharpCode.ILSpy.TextView
 		{
 			WriteIndentIfNeeded();
 			int start = builder.Length;
-			var name = omitSuffix ? opCode.Name.TrimEnd('.') : opCode.Name;
+			string name = opCode.Name;
+			if (omitSuffix)
+			{
+				int lastDot = name.LastIndexOf('.');
+				if (lastDot > 0)
+					name = name.Remove(lastDot + 1);
+			}
 			builder.Append(name);
 			References.Add(new ReferenceSegment {
 				StartOffset = start,