Allow mouse wheel scrolling in the Shelf

Copy/Shelf/ShelfRootView.swift

@@ -644,6 +644,7 @@ private struct ShelfItemsRow: View {
                     // Gives the drag-select the row's scroll view, which it scrolls while
                     // the pointer is held at an edge.
                     .background(CardRowScrollViewFinder { viewModel.cardSweep.scrollView = $0 })
+                    .background(ShelfWheelScrollBridge())
                     .coordinateSpace(name: CardSweep.contentSpace)
                     .onGeometryChange(for: CGPoint.self) {
                         $0.frame(in: .named(CardSweep.viewportSpace)).origin

Copy/Shelf/ShelfWheelScroll.swift

@@ -0,0 +1,50 @@
+import AppKit
+import SwiftUI
+
+enum ShelfWheelScroll {
+    static func horizontalOffset(horizontal: CGFloat, vertical: CGFloat) -> CGFloat? {
+        guard horizontal == 0, vertical != 0 else { return nil }
+        return -vertical
+    }
+}
+
+struct ShelfWheelScrollBridge: NSViewRepresentable {
+    func makeNSView(context: Context) -> NSView { ShelfWheelScrollView() }
+
+    func updateNSView(_ nsView: NSView, context: Context) {}
+}
+
+private final class ShelfWheelScrollView: NSView {
+    private var monitor: Any?
+
+    override func viewDidMoveToWindow() {
+        super.viewDidMoveToWindow()
+        installMonitor()
+    }
+
+    deinit {
+        if let monitor { NSEvent.removeMonitor(monitor) }
+    }
+
+    private func installMonitor() {
+        guard monitor == nil else { return }
+        monitor = NSEvent.addLocalMonitorForEvents(matching: .scrollWheel) { [weak self] event in
+            guard let self, let scroll = self.enclosingScrollView,
+                  event.window === self.window,
+                  let offset = ShelfWheelScroll.horizontalOffset(
+                    horizontal: event.scrollingDeltaX, vertical: event.scrollingDeltaY),
+                  self.contains(event: event, in: scroll) else { return event }
+
+            let maximum = max(0, (scroll.documentView?.frame.width ?? 0) - scroll.contentSize.width)
+            let x = min(max(0, scroll.contentView.bounds.origin.x + offset), maximum)
+            scroll.contentView.scroll(to: NSPoint(x: x, y: scroll.contentView.bounds.origin.y))
+            scroll.reflectScrolledClipView(scroll.contentView)
+            return nil
+        }
+    }
+
+    private func contains(event: NSEvent, in scroll: NSScrollView) -> Bool {
+        let point = scroll.contentView.convert(event.locationInWindow, from: nil)
+        return scroll.contentView.bounds.contains(point)
+    }
+}

CopyTests/ShelfWheelScrollTests.swift

@@ -0,0 +1,20 @@
+import AppKit
+import XCTest
+
+final class ShelfWheelScrollTests: XCTestCase {
+    func testDownwardWheelMovementBecomesRightwardShelfMovement() {
+        XCTAssertEqual(ShelfWheelScroll.horizontalOffset(horizontal: 0, vertical: -3), 3)
+    }
+
+    func testUpwardWheelMovementBecomesLeftwardShelfMovement() {
+        XCTAssertEqual(ShelfWheelScroll.horizontalOffset(horizontal: 0, vertical: 3), -3)
+    }
+
+    func testHorizontalTrackpadMovementStaysWithAppKit() {
+        XCTAssertNil(ShelfWheelScroll.horizontalOffset(horizontal: 2, vertical: 0))
+    }
+
+    func testCombinedAxisTrackpadMovementStaysWithAppKit() {
+        XCTAssertNil(ShelfWheelScroll.horizontalOffset(horizontal: 2, vertical: -3))
+    }
+}

project.yml

@@ -71,6 +71,7 @@ targets:
     sources:
       - CopyTests
       - Copy/PasteStack/PasteStackModel.swift
+      - Copy/Shelf/ShelfWheelScroll.swift
       - Copy/PasteStack/PasteStackBatchConfirmation.swift
     dependencies:
       - package: CopyCore