Only disable color when NO_COLOR is non-empty

DevProxy.Tests/DevProxyCommandNoColorTests.cs

@@ -0,0 +1,37 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using DevProxy.Commands;
+
+using Xunit;
+
+namespace DevProxy.Tests;
+
+public class DevProxyCommandNoColorTests
+{
+    [Theory]
+    [InlineData(null, false)]
+    [InlineData("", false)]
+    [InlineData("1", true)]
+    [InlineData("true", true)]
+    public void ShouldDisableColor_RespectsNoColorEnv(string? noColor, bool expected)
+    {
+        Assert.Equal(expected, DevProxyCommand.ShouldDisableColor([], noColor, null));
+    }
+
+    [Fact]
+    public void ShouldDisableColor_NoColorOption_DisablesColor()
+    {
+        Assert.True(DevProxyCommand.ShouldDisableColor(["devproxy", "--no-color"], "", null));
+    }
+
+    [Theory]
+    [InlineData("dumb", true)]
+    [InlineData("DUMB", true)]
+    [InlineData("xterm-256color", false)]
+    public void ShouldDisableColor_RespectsTerm(string term, bool expected)
+    {
+        Assert.Equal(expected, DevProxyCommand.ShouldDisableColor([], null, term));
+    }
+}

DevProxy/Commands/DevProxyCommand.cs

@@ -231,15 +231,21 @@ public static bool NoColor
                 return field;
             }
 
-            var args = Environment.GetCommandLineArgs();
-            field = args.Contains(NoColorOptionName) ||
-                Environment.GetEnvironmentVariable("NO_COLOR") is not null ||
-                string.Equals(Environment.GetEnvironmentVariable("TERM"), "dumb", StringComparison.OrdinalIgnoreCase);
+            field = ShouldDisableColor(
+                Environment.GetCommandLineArgs(),
+                Environment.GetEnvironmentVariable("NO_COLOR"),
+                Environment.GetEnvironmentVariable("TERM"));
             _noColorResolved = true;
             return field;
         }
     }
 
+    // Per https://no-color.org, NO_COLOR disables color only when present and not empty.
+    internal static bool ShouldDisableColor(string[] args, string? noColorEnv, string? termEnv) =>
+        args.Contains(NoColorOptionName) ||
+        !string.IsNullOrEmpty(noColorEnv) ||
+        string.Equals(termEnv, "dumb", StringComparison.OrdinalIgnoreCase);
+
     public DevProxyCommand(
         IEnumerable<IPlugin> plugins,
         ISet<UrlToWatch> urlsToWatch,