Add revertable diff interactions and hover affordance controls

.github/workflows/pr-tests.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Select latest stable Xcode
         uses: maxim-lobanov/setup-xcode@v1
         with:
-          xcode-version: "latest-stable"
+          xcode-version: "16.4"
 
       - name: Install xcsift
         run: brew install xcsift
@@ -42,7 +42,7 @@ jobs:
       - name: Select latest stable Xcode
         uses: maxim-lobanov/setup-xcode@v1
         with:
-          xcode-version: "latest-stable"
+          xcode-version: "16.4"
 
       - name: Install xcsift
         run: brew install xcsift
@@ -53,6 +53,10 @@ jobs:
           mkdir -p "$SNAPSHOT_ARTIFACTS"
           swift test --filter TextDiffSnapshotTests 2>&1 | tee "$SNAPSHOT_ARTIFACTS/swift-test.log" | xcsift --warnings
 
+      - name: Collect snapshot artifact bundle
+        if: always()
+        run: swift Scripts/collect_snapshot_artifacts.swift
+
       - name: Upload snapshot artifacts
         if: always()
         uses: actions/upload-artifact@v4

.swiftpm/xcode/xcshareddata/xcbaselines/TextDiffTests.xcbaseline/356ED75C-DC2E-441F-90EF-CD873A212B51.plist

@@ -0,0 +1,42 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>classNames</key>
+	<dict>
+		<key>DiffLayouterPerformanceTests</key>
+		<dict>
+			<key>testLayoutPerformance1000Words()</key>
+			<dict>
+				<key>com.apple.dt.XCTMetric_Clock.time.monotonic</key>
+				<dict>
+					<key>baselineAverage</key>
+					<real>0.0168714</real>
+					<key>baselineIntegrationDisplayName</key>
+					<string>Local Baseline</string>
+				</dict>
+			</dict>
+			<key>testLayoutPerformance200Words()</key>
+			<dict>
+				<key>com.apple.dt.XCTMetric_Clock.time.monotonic</key>
+				<dict>
+					<key>baselineAverage</key>
+					<real>0.003373</real>
+					<key>baselineIntegrationDisplayName</key>
+					<string>Local Baseline</string>
+				</dict>
+			</dict>
+			<key>testLayoutPerformance500Words()</key>
+			<dict>
+				<key>com.apple.dt.XCTMetric_Clock.time.monotonic</key>
+				<dict>
+					<key>baselineAverage</key>
+					<real>0.0082662</real>
+					<key>baselineIntegrationDisplayName</key>
+					<string>Local Baseline</string>
+				</dict>
+			</dict>
+		</dict>
+	</dict>
+</dict>
+</plist>

.swiftpm/xcode/xcshareddata/xcbaselines/TextDiffTests.xcbaseline/Info.plist

@@ -28,6 +28,30 @@
 			<key>targetArchitecture</key>
 			<string>arm64</string>
 		</dict>
+		<key>356ED75C-DC2E-441F-90EF-CD873A212B51</key>
+		<dict>
+			<key>localComputer</key>
+			<dict>
+				<key>busSpeedInMHz</key>
+				<integer>0</integer>
+				<key>cpuCount</key>
+				<integer>1</integer>
+				<key>cpuKind</key>
+				<string>Apple M5 Pro</string>
+				<key>cpuSpeedInMHz</key>
+				<integer>0</integer>
+				<key>logicalCPUCoresPerPackage</key>
+				<integer>18</integer>
+				<key>modelCode</key>
+				<string>Mac17,9</string>
+				<key>physicalCPUCoresPerPackage</key>
+				<integer>18</integer>
+				<key>platformIdentifier</key>
+				<string>com.apple.platform.macosx</string>
+			</dict>
+			<key>targetArchitecture</key>
+			<string>arm64</string>
+		</dict>
 	</dict>
 </dict>
 </plist>

AGENTS.md

@@ -34,3 +34,32 @@ Use the flags above (e.g., `--coverage-details`, `--coverage-path`) as needed, b
 ## Code search
 - Use [`ripgrep`](https://github.com/BurntSushi/ripgrep) (`rg`) for searching within files—it is much faster than grep/ack/ag, respects `.gitignore`, and has smart defaults.
 - Typical commands: `rg "TODO"` (find TODOs), `rg -n --glob '!dist' pattern.swift` (search with line numbers while excluding `dist`).
+
+<!-- BACKLOG.MD MCP GUIDELINES START -->
+
+<CRITICAL_INSTRUCTION>
+
+## BACKLOG WORKFLOW INSTRUCTIONS
+
+This project uses Backlog.md MCP for all task and project management activities.
+
+**CRITICAL GUIDANCE**
+
+- If your client supports MCP resources, read `backlog://workflow/overview` to understand when and how to use Backlog for this project.
+- If your client only supports tools or the above request fails, call `backlog.get_workflow_overview()` tool to load the tool-oriented overview (it lists the matching guide tools).
+
+- **First time working here?** Read the overview resource IMMEDIATELY to learn the workflow
+- **Already familiar?** You should have the overview cached ("## Backlog.md Overview (MCP)")
+- **When to read it**: BEFORE creating tasks, or when you're unsure whether to track work
+
+These guides cover:
+- Decision framework for when to create tasks
+- Search-first workflow to avoid duplicates
+- Links to detailed guides for task creation, execution, and finalization
+- MCP tools reference
+
+You MUST read the overview resource to understand the complete workflow. The information is NOT summarized here.
+
+</CRITICAL_INSTRUCTION>
+
+<!-- BACKLOG.MD MCP GUIDELINES END -->

README.md

@@ -154,3 +154,17 @@ Update baselines intentionally:
 2. Run `swift test 2>&1 | xcsift --quiet` once to rewrite baselines.
 3. Switch the suite trait back to `.missing`.
 4. Review snapshot image diffs in your PR before merging.
+
+## Performance Testing
+
+- Performance baselines for `DiffLayouterPerformanceTests` are stored under `.swiftpm/xcode/xcshareddata/xcbaselines/TextDiffTests.xcbaseline/`.
+- `swift test` runs the performance tests, but it does not surface the committed Xcode baseline values in its output.
+- For baseline-aware runs, use the generated SwiftPM workspace and the `TextDiff` scheme.
+
+Run the layouter performance suite with Xcode:
+
+```bash
+xcodebuild -workspace .swiftpm/xcode/package.xcworkspace -scheme TextDiff -destination 'platform=macOS' -configuration Debug test -only-testing:TextDiffTests/DiffLayouterPerformanceTests 2>&1 | xcsift
+```
+
+If you need the raw measured averages for comparison, run the same command once without `xcsift` because XCTest prints the per-test values directly in the plain `xcodebuild` output.

Scripts/collect_snapshot_artifacts.swift

@@ -0,0 +1,107 @@
+#!/usr/bin/env swift
+
+import AppKit
+import CoreImage
+import Foundation
+
+let fileManager = FileManager.default
+let repoRoot = URL(fileURLWithPath: fileManager.currentDirectoryPath, isDirectory: true)
+let artifactsURL = URL(
+    fileURLWithPath: ProcessInfo.processInfo.environment["SNAPSHOT_ARTIFACTS"]
+        ?? repoRoot.appendingPathComponent("snapshot-artifacts", isDirectory: true).path,
+    isDirectory: true
+)
+let referencesRootURL = repoRoot
+    .appendingPathComponent("Tests", isDirectory: true)
+    .appendingPathComponent("TextDiffTests", isDirectory: true)
+    .appendingPathComponent("__Snapshots__", isDirectory: true)
+
+guard fileManager.fileExists(atPath: artifactsURL.path) else {
+    print("No snapshot artifacts directory found at \(artifactsURL.path)")
+    exit(0)
+}
+
+let ciContext = CIContext(options: nil)
+let pngExtension = "png"
+
+func loadCIImage(from url: URL) -> CIImage? {
+    guard let image = NSImage(contentsOf: url),
+          let tiff = image.tiffRepresentation,
+          let bitmap = NSBitmapImageRep(data: tiff) else {
+        return nil
+    }
+    return CIImage(bitmapImageRep: bitmap)
+}
+
+func writePNG(ciImage: CIImage, to url: URL) throws {
+    let extent = ciImage.extent.integral
+    guard let colorSpace = CGColorSpace(name: CGColorSpace.sRGB),
+          let cgImage = ciContext.createCGImage(ciImage, from: extent, format: .RGBA8, colorSpace: colorSpace) else {
+        throw NSError(domain: "collect_snapshot_artifacts", code: 1)
+    }
+
+    let rep = NSBitmapImageRep(cgImage: cgImage)
+    guard let data = rep.representation(using: .png, properties: [:]) else {
+        throw NSError(domain: "collect_snapshot_artifacts", code: 2)
+    }
+    try data.write(to: url)
+}
+
+func diffImage(referenceURL: URL, failedURL: URL) -> CIImage? {
+    guard let reference = loadCIImage(from: referenceURL),
+          let failed = loadCIImage(from: failedURL),
+          let filter = CIFilter(name: "CIDifferenceBlendMode") else {
+        return nil
+    }
+    filter.setValue(reference, forKey: kCIInputImageKey)
+    filter.setValue(failed, forKey: kCIInputBackgroundImageKey)
+    return filter.outputImage
+}
+
+let artifactFiles = fileManager.enumerator(
+    at: artifactsURL,
+    includingPropertiesForKeys: [.isRegularFileKey],
+    options: [.skipsHiddenFiles]
+) ?? NSEnumerator()
+
+var enrichedCount = 0
+
+for case let fileURL as URL in artifactFiles {
+    guard fileURL.pathExtension == pngExtension else { continue }
+
+    let relativePath = fileURL.path.replacingOccurrences(of: artifactsURL.path + "/", with: "")
+    guard !relativePath.contains(".reference."),
+          !relativePath.contains(".failed."),
+          !relativePath.contains(".diff.") else {
+        continue
+    }
+
+    let referenceURL = referencesRootURL.appendingPathComponent(relativePath)
+    guard fileManager.fileExists(atPath: referenceURL.path) else {
+        continue
+    }
+
+    let baseURL = fileURL.deletingPathExtension()
+    let failedCopyURL = baseURL.appendingPathExtension("failed").appendingPathExtension(pngExtension)
+    let referenceCopyURL = baseURL.appendingPathExtension("reference").appendingPathExtension(pngExtension)
+    let diffURL = baseURL.appendingPathExtension("diff").appendingPathExtension(pngExtension)
+
+    if fileManager.fileExists(atPath: failedCopyURL.path) {
+        try fileManager.removeItem(at: fileURL)
+    } else {
+        try fileManager.moveItem(at: fileURL, to: failedCopyURL)
+    }
+
+    if !fileManager.fileExists(atPath: referenceCopyURL.path) {
+        try fileManager.copyItem(at: referenceURL, to: referenceCopyURL)
+    }
+
+    if !fileManager.fileExists(atPath: diffURL.path),
+       let diff = diffImage(referenceURL: referenceURL, failedURL: failedCopyURL) {
+        try writePNG(ciImage: diff, to: diffURL)
+    }
+
+    enrichedCount += 1
+}
+
+print("Enriched \(enrichedCount) snapshot artifact bundle(s) in \(artifactsURL.path)")