fix: restore cross-platform discovery heuristics

Author: mikecarr

Companion.Tests/Services/OpenIpcDiscoveryServiceTests.cs

@@ -12,6 +12,12 @@ public class OpenIpcDiscoveryServiceTests
     [Test]
     public void BuildDiscoveryScanPrefix_ClampsBroadSubnetToClassC()
     {
+        if (!OperatingSystem.IsWindows())
+        {
+            Assert.Pass("Windows-specific discovery clamp.");
+            return;
+        }
+
         var prefix = NetworkHelper.BuildDiscoveryScanPrefix(
             IPAddress.Parse("192.168.2.45"),
             IPAddress.Parse("255.255.0.0"));
@@ -55,4 +61,47 @@ public void SelectPreferredCandidates_PrefersUsbCandidatesOverVirtualNoGatewayAd
         Assert.That(selected.Select(candidate => candidate.InterfaceName).ToArray(),
             Is.EqualTo(new[] { "USB Ethernet/RNDIS Gadget" }));
     }
+
+    [Test]
+    public void SelectPreferredCandidates_OnMacOsPrefersBridge100ForInternetSharing()
+    {
+        if (!OperatingSystem.IsMacOS())
+        {
+            Assert.Pass("macOS-specific bridge selection.");
+            return;
+        }
+
+        var method = typeof(OpenIpcDiscoveryService).GetMethod(
+            "SelectPreferredCandidates",
+            BindingFlags.NonPublic | BindingFlags.Static);
+
+        Assert.That(method, Is.Not.Null);
+
+        var candidates = new List<NetworkHelper.LocalNetworkCandidate>
+        {
+            new(
+                IPAddress.Parse("192.168.1.55"),
+                IPAddress.Parse("255.255.255.0"),
+                "en17",
+                60,
+                false,
+                false,
+                false,
+                true),
+            new(
+                IPAddress.Parse("192.168.2.1"),
+                IPAddress.Parse("255.255.255.0"),
+                "bridge100",
+                35,
+                false,
+                false,
+                false,
+                true)
+        };
+
+        var selected = (IReadOnlyList<NetworkHelper.LocalNetworkCandidate>)method!.Invoke(null, new object[] { candidates })!;
+
+        Assert.That(selected.Select(candidate => candidate.InterfaceName).ToArray(),
+            Is.EqualTo(new[] { "bridge100" }));
+    }
 }

Companion/Services/NetworkHelper.cs

@@ -133,6 +133,9 @@ public static string BuildScanPrefix(IPAddress ipAddress, IPAddress mask)
 
     public static string BuildDiscoveryScanPrefix(IPAddress ipAddress, IPAddress mask)
     {
+        if (!OperatingSystem.IsWindows())
+            return BuildScanPrefix(ipAddress, mask);
+
         var prefix = BuildScanPrefix(ipAddress, mask);
         var parts = prefix.Split('.', StringSplitOptions.RemoveEmptyEntries);
 
@@ -226,16 +229,40 @@ private static bool IsUsbLikeInterface(NetworkInterface nic)
     private static bool IsVirtualLikeInterface(NetworkInterface nic)
     {
         var descriptor = $"{nic.Name} {nic.Description} {nic.Id}";
+        if (OperatingSystem.IsWindows())
+        {
+            return ContainsAny(descriptor,
+                "virtual",
+                "vmware",
+                "hyper-v",
+                "vbox",
+                "docker",
+                "wsl",
+                "vethernet",
+                "default switch",
+                "tailscale",
+                "zerotier");
+        }
+
+        if (OperatingSystem.IsMacOS())
+        {
+            if (nic.Name.Equals("bridge100", StringComparison.OrdinalIgnoreCase))
+                return false;
+
+            return ContainsAny(descriptor,
+                "virtual",
+                "vmware",
+                "vbox",
+                "docker",
+                "tailscale",
+                "zerotier");
+        }
+
         return ContainsAny(descriptor,
-            "bridge",
             "virtual",
             "vmware",
-            "hyper-v",
             "vbox",
             "docker",
-            "wsl",
-            "vethernet",
-            "default switch",
             "tailscale",
             "zerotier");
     }

Companion/Services/OpenIpcDiscoveryService.cs

@@ -187,6 +187,17 @@ private async Task<bool> TryPingAsync(string host, CancellationToken cancellatio
         if (usbCandidates.Count > 0)
             return usbCandidates;
 
+        if (OperatingSystem.IsMacOS())
+        {
+            var macBridgeCandidates = candidates
+                .Where(candidate => candidate.InterfaceName.Equals("bridge100", StringComparison.OrdinalIgnoreCase) &&
+                                    candidate.IsPrivateIPv4)
+                .ToList();
+
+            if (macBridgeCandidates.Count > 0)
+                return macBridgeCandidates;
+        }
+
         var directAttached = candidates
             .Where(candidate => !candidate.HasGateway &&
                                 !candidate.IsVirtualLike &&