fix(editor): restore viewport after mouse click scroll

Sources/SaplingEditor/HybridMarkdownEditor.swift

@@ -209,7 +209,7 @@ private struct NativeMarkdownTextView: NSViewRepresentable {
             coordinator?.applyHybridAttributes(to: textView, cause: .focusChange)
         }
         textView.onUserEditingInteraction = { [weak coordinator = context.coordinator] textView, interactionKind in
-            coordinator?.recordUserEditingInteraction(interactionKind)
+            coordinator?.recordUserEditingInteraction(interactionKind, in: textView)
         }
         textView.isRichText = false
         textView.isEditable = true
@@ -274,6 +274,7 @@ private struct NativeMarkdownTextView: NSViewRepresentable {
         fileprivate var viewportStabilityEvents: [ViewportStabilityEvent] = []
         private var lastUserInteractionKind: EditorInteractionKind = .programmatic
         private var pendingSelectionInteractionKind: EditorInteractionKind?
+        private var pendingMouseVisibleOrigin: NSPoint?
 
         init(_ parent: NativeMarkdownTextView) {
             self.parent = parent
@@ -320,6 +321,11 @@ private struct NativeMarkdownTextView: NSViewRepresentable {
             if interactionKind != .programmatic {
                 hasUserActivatedEditing = true
             }
+            defer {
+                if interactionKind == .mouse {
+                    pendingMouseVisibleOrigin = nil
+                }
+            }
             applyHybridAttributes(to: textView, cause: .selectionChange(interactionKind))
             parent.selection = newSelection
         }
@@ -345,7 +351,8 @@ private struct NativeMarkdownTextView: NSViewRepresentable {
                 ? ViewportPresentationAnchor.capture(
                     in: textView,
                     selectedRange: selectedRange,
-                    lineIndex: currentLineIndex
+                    lineIndex: currentLineIndex,
+                    visibleOrigin: visibleOriginOverride(for: cause)
                 )
                 : nil
             let start = Date()
@@ -420,8 +427,13 @@ private struct NativeMarkdownTextView: NSViewRepresentable {
             applyHybridAttributes(to: textView, cause: .editingActivation(interactionKind))
         }
 
-        func recordUserEditingInteraction(_ interactionKind: EditorInteractionKind) {
+        func recordUserEditingInteraction(_ interactionKind: EditorInteractionKind, in textView: NSTextView) {
             lastUserInteractionKind = interactionKind
+            if interactionKind == .mouse {
+                pendingMouseVisibleOrigin = textView.enclosingScrollView?.contentView.bounds.origin
+            } else {
+                pendingMouseVisibleOrigin = nil
+            }
         }
 
         func setSelection(
@@ -452,6 +464,11 @@ private struct NativeMarkdownTextView: NSViewRepresentable {
             return !visibleRect.contains(point)
         }
 
+        private func visibleOriginOverride(for cause: PresentationUpdateCause) -> NSPoint? {
+            guard case .selectionChange(.mouse) = cause else { return nil }
+            return pendingMouseVisibleOrigin
+        }
+
         func performProgrammaticUpdate(_ updates: () -> Void) {
             programmaticUpdateDepth += 1
             defer {
@@ -783,7 +800,8 @@ private struct ViewportPresentationAnchor {
     static func capture(
         in textView: NSTextView,
         selectedRange: NSRange,
-        lineIndex: DocumentLineIndex
+        lineIndex: DocumentLineIndex,
+        visibleOrigin visibleOriginOverride: NSPoint? = nil
     ) -> ViewportPresentationAnchor? {
         guard let scrollView = textView.enclosingScrollView,
               textView.string.utf16.count > 0
@@ -796,7 +814,7 @@ private struct ViewportPresentationAnchor {
         )
         guard let point = textView.presentationAnchorPoint(at: location) else { return nil }
 
-        let visibleOrigin = scrollView.contentView.bounds.origin
+        let visibleOrigin = visibleOriginOverride ?? scrollView.contentView.bounds.origin
         return ViewportPresentationAnchor(
             characterLocation: location,
             visibleOrigin: visibleOrigin,
@@ -1130,7 +1148,7 @@ public final class HybridMarkdownLiveEditorHarness {
             coordinator?.applyHybridAttributes(to: textView, cause: .focusChange)
         }
         self.textView.onUserEditingInteraction = { [weak coordinator] textView, interactionKind in
-            coordinator?.recordUserEditingInteraction(interactionKind)
+            coordinator?.recordUserEditingInteraction(interactionKind, in: textView)
         }
         self.textView.string = source
         self.textView.delegate = coordinator
@@ -1192,11 +1210,19 @@ public final class HybridMarkdownLiveEditorHarness {
         setSelection(range, interactionKind: .mouse)
     }
 
+    public func simulateMouseSelectionAfterNativeScroll(_ range: NSRange, nativeScrollY: CGFloat) {
+        coordinator.recordUserEditingInteraction(.mouse, in: textView)
+        scrollViewportWithoutSync(toY: nativeScrollY)
+        textView.setSelectedRange(range)
+        coordinator.textViewDidChangeSelection(Notification(name: NSTextView.didChangeSelectionNotification, object: textView))
+        syncState()
+    }
+
     private func setSelection(_ range: NSRange, interactionKind: EditorInteractionKind) {
         if interactionKind == .programmatic {
             coordinator.activateEditingPresentation(in: textView, interactionKind: interactionKind)
         } else {
-            coordinator.recordUserEditingInteraction(interactionKind)
+            coordinator.recordUserEditingInteraction(interactionKind, in: textView)
         }
         coordinator.setSelection(range, in: textView, interactionKind: interactionKind)
         coordinator.textViewDidChangeSelection(Notification(name: NSTextView.didChangeSelectionNotification, object: textView))
@@ -1220,11 +1246,15 @@ public final class HybridMarkdownLiveEditorHarness {
     }
 
     public func scrollViewport(toY y: CGFloat) {
+        scrollViewportWithoutSync(toY: y)
+        syncState()
+    }
+
+    private func scrollViewportWithoutSync(toY y: CGFloat) {
         let maxY = max(0, textView.bounds.height - scrollView.contentView.bounds.height)
         let target = NSPoint(x: 0, y: max(0, min(y, maxY)))
         scrollView.contentView.scroll(to: target)
         scrollView.reflectScrolledClipView(scrollView.contentView)
-        syncState()
     }
 
     public func viewportOrigin() -> CGPoint {

Tests/SaplingEditorTests/HybridMarkdownLiveEditorHarnessTests.swift

@@ -27,6 +27,22 @@ final class HybridMarkdownLiveEditorHarnessTests: XCTestCase {
         XCTAssertEqual(harness.viewportOrigin().y, 0, accuracy: 0.001)
     }
 
+    func testNativeMouseScrollDuringTopClickIsRestored() {
+        let source = (["# Heading", "Opening paragraph"] + (1...80).map { "Line \($0)" })
+            .joined(separator: "\n")
+        let harness = HybridMarkdownLiveEditorHarness(source: source)
+        harness.simulateLaunchFirstResponder()
+        harness.scrollViewport(toY: 0)
+
+        let paragraphLocation = (source as NSString).range(of: "Opening").location
+        harness.simulateMouseSelectionAfterNativeScroll(
+            NSRange(location: paragraphLocation, length: 0),
+            nativeScrollY: 48
+        )
+
+        XCTAssertEqual(harness.viewportOrigin().y, 0, accuracy: 0.001)
+    }
+
     func testLiveParagraphGeometryReturnsAfterClickAndFocusAway() throws {
         let source = """
         # Heading