macos: show release notes link

macos/Sources/Features/Update/UpdatePopoverView.swift

@@ -191,6 +191,28 @@ fileprivate struct UpdateAvailableView: View {
                 }
             }
             .padding(16)
+            
+            if let notes = update.releaseNotes {
+                Divider()
+                
+                Link(destination: notes.url) {
+                    HStack {
+                        Image(systemName: "doc.text")
+                            .font(.system(size: 11))
+                        Text(notes.label)
+                            .font(.system(size: 11, weight: .medium))
+                        Spacer()
+                        Image(systemName: "arrow.up.right")
+                            .font(.system(size: 10))
+                    }
+                    .foregroundColor(.primary)
+                    .padding(12)
+                    .frame(maxWidth: .infinity)
+                    .background(Color(nsColor: .controlBackgroundColor))
+                    .contentShape(Rectangle())
+                }
+                .buttonStyle(.plain)
+            }
         }
     }
 }

macos/Sources/Features/Update/UpdateViewModel.swift

@@ -173,6 +173,76 @@ enum UpdateState: Equatable {
     struct UpdateAvailable {
         let appcastItem: SUAppcastItem
         let reply: @Sendable (SPUUserUpdateChoice) -> Void
+        
+        var releaseNotes: ReleaseNotes? {
+            let currentCommit = Bundle.main.infoDictionary?["GhosttyCommit"] as? String
+            return ReleaseNotes(displayVersionString: appcastItem.displayVersionString, currentCommit: currentCommit)
+        }
+    }
+    
+    enum ReleaseNotes {
+        case commit(URL)
+        case compareTip(URL)
+        case tagged(URL)
+        
+        init?(displayVersionString: String, currentCommit: String?) {
+            let version = displayVersionString
+            
+            // Check for semantic version (x.y.z)
+            if let semver = Self.extractSemanticVersion(from: version) {
+                let slug = semver.replacingOccurrences(of: ".", with: "-")
+                if let url = URL(string: "https://ghostty.org/docs/install/release-notes/\(slug)") {
+                    self = .tagged(url)
+                    return
+                }
+            }
+            
+            // Fall back to git hash detection
+            guard let newHash = Self.extractGitHash(from: version) else {
+                return nil
+            }
+            
+            if let currentHash = currentCommit, !currentHash.isEmpty,
+               let url = URL(string: "https://github.com/ghostty-org/ghostty/compare/\(currentHash)...\(newHash)") {
+                self = .compareTip(url)
+            } else if let url = URL(string: "https://github.com/ghostty-org/ghostty/commit/\(newHash)") {
+                self = .commit(url)
+            } else {
+                return nil
+            }
+        }
+        
+        private static func extractSemanticVersion(from version: String) -> String? {
+            let pattern = #"^\d+\.\d+\.\d+$"#
+            if version.range(of: pattern, options: .regularExpression) != nil {
+                return version
+            }
+            return nil
+        }
+        
+        private static func extractGitHash(from version: String) -> String? {
+            let pattern = #"[0-9a-f]{7,40}"#
+            if let range = version.range(of: pattern, options: .regularExpression) {
+                return String(version[range])
+            }
+            return nil
+        }
+        
+        var url: URL {
+            switch self {
+            case .commit(let url): return url
+            case .compareTip(let url): return url
+            case .tagged(let url): return url
+            }
+        }
+        
+        var label: String {
+            switch (self) {
+            case .commit: return "View GitHub Commit"
+            case .compareTip: return "Changes Since This Tip Release"
+            case .tagged: return "View Release Notes"
+            }
+        }
     }
     
     struct Error {

macos/Tests/Update/ReleaseNotesTests.swift

@@ -0,0 +1,130 @@
+import Testing
+import Foundation
+@testable import Ghostty
+
+struct ReleaseNotesTests {
+    /// Test tagged release (semantic version)
+    @Test func testTaggedRelease() async throws {
+        let notes = UpdateState.ReleaseNotes(
+            displayVersionString: "1.2.3",
+            currentCommit: nil
+        )
+        
+        #expect(notes != nil)
+        if case .tagged(let url) = notes {
+            #expect(url.absoluteString == "https://ghostty.org/docs/install/release-notes/1-2-3")
+            #expect(notes?.label == "View Release Notes")
+        } else {
+            Issue.record("Expected tagged case")
+        }
+    }
+    
+    /// Test tip release comparison with current commit
+    @Test func testTipReleaseComparison() async throws {
+        let notes = UpdateState.ReleaseNotes(
+            displayVersionString: "tip-abc1234",
+            currentCommit: "def5678"
+        )
+        
+        #expect(notes != nil)
+        if case .compareTip(let url) = notes {
+            #expect(url.absoluteString == "https://github.com/ghostty-org/ghostty/compare/def5678...abc1234")
+            #expect(notes?.label == "Changes Since This Tip Release")
+        } else {
+            Issue.record("Expected compareTip case")
+        }
+    }
+    
+    /// Test tip release without current commit
+    @Test func testTipReleaseWithoutCurrentCommit() async throws {
+        let notes = UpdateState.ReleaseNotes(
+            displayVersionString: "tip-abc1234",
+            currentCommit: nil
+        )
+        
+        #expect(notes != nil)
+        if case .commit(let url) = notes {
+            #expect(url.absoluteString == "https://github.com/ghostty-org/ghostty/commit/abc1234")
+            #expect(notes?.label == "View GitHub Commit")
+        } else {
+            Issue.record("Expected commit case")
+        }
+    }
+    
+    /// Test tip release with empty current commit
+    @Test func testTipReleaseWithEmptyCurrentCommit() async throws {
+        let notes = UpdateState.ReleaseNotes(
+            displayVersionString: "tip-abc1234",
+            currentCommit: ""
+        )
+        
+        #expect(notes != nil)
+        if case .commit(let url) = notes {
+            #expect(url.absoluteString == "https://github.com/ghostty-org/ghostty/commit/abc1234")
+        } else {
+            Issue.record("Expected commit case")
+        }
+    }
+    
+    /// Test version with full 40-character hash
+    @Test func testFullGitHash() async throws {
+        let notes = UpdateState.ReleaseNotes(
+            displayVersionString: "tip-1234567890abcdef1234567890abcdef12345678",
+            currentCommit: nil
+        )
+        
+        #expect(notes != nil)
+        if case .commit(let url) = notes {
+            #expect(url.absoluteString == "https://github.com/ghostty-org/ghostty/commit/1234567890abcdef1234567890abcdef12345678")
+        } else {
+            Issue.record("Expected commit case")
+        }
+    }
+    
+    /// Test version with no recognizable pattern
+    @Test func testInvalidVersion() async throws {
+        let notes = UpdateState.ReleaseNotes(
+            displayVersionString: "unknown-version",
+            currentCommit: nil
+        )
+        
+        #expect(notes == nil)
+    }
+    
+    /// Test semantic version with prerelease suffix should not match
+    @Test func testSemanticVersionWithSuffix() async throws {
+        let notes = UpdateState.ReleaseNotes(
+            displayVersionString: "1.2.3-beta",
+            currentCommit: nil
+        )
+        
+        // Should not match semantic version pattern, falls back to hash detection
+        #expect(notes == nil)
+    }
+    
+    /// Test semantic version with 4 components should not match
+    @Test func testSemanticVersionFourComponents() async throws {
+        let notes = UpdateState.ReleaseNotes(
+            displayVersionString: "1.2.3.4",
+            currentCommit: nil
+        )
+        
+        // Should not match pattern
+        #expect(notes == nil)
+    }
+    
+    /// Test version string with git hash embedded
+    @Test func testVersionWithEmbeddedHash() async throws {
+        let notes = UpdateState.ReleaseNotes(
+            displayVersionString: "v2024.01.15-abc1234",
+            currentCommit: "def5678"
+        )
+        
+        #expect(notes != nil)
+        if case .compareTip(let url) = notes {
+            #expect(url.absoluteString == "https://github.com/ghostty-org/ghostty/compare/def5678...abc1234")
+        } else {
+            Issue.record("Expected compareTip case")
+        }
+    }
+}