An NSClipView subclass which performs drag-scrolling.

gistfile1.txt

class DraggableClipView: NSClipView
{
    private var clickPoint: NSPoint!
    private var originalOrigin: NSPoint!

    override func mouseDown(event: NSEvent) {
        clickPoint = event.locationInWindow
        originalOrigin = bounds.origin
    }

    override func mouseDragged(event: NSEvent) {
        // Account for a magnified parent scrollview.
        let scale = (superview as? NSScrollView)?.magnification ?? 1.0
        let newPoint = event.locationInWindow
        let newOrigin = NSPoint(x: originalOrigin.x + (clickPoint.x - newPoint.x) / scale,
                                y: originalOrigin.y + (clickPoint.y - newPoint.y) / scale)
        let constrainedRect = constrainBoundsRect(NSRect(origin: newOrigin, size: bounds.size))
        scrollToPoint(constrainedRect.origin)
        superview?.reflectScrolledClipView(self)
    }

    override func mouseUp(event: NSEvent) {
        clickPoint = nil
    }
}

Really awesome! Thank you a lot for that!

For my needs I modified it for Swift 5 and inverted the Y axis.

class DraggableClipView: NSClipView {

    private var clickPoint: NSPoint!
    private var originalOrigin: NSPoint!

    override func mouseDown(with event: NSEvent) {
        clickPoint = event.locationInWindow
        originalOrigin = bounds.origin
    }

    override func mouseDragged(with event: NSEvent) {

        let scale = (superview as? NSScrollView)?.magnification ?? 1.0

        let newPoint = event.locationInWindow

        let newOrigin = NSPoint(x: originalOrigin.x + (clickPoint.x - newPoint.x) / scale,
                                y: originalOrigin.y - (clickPoint.y - newPoint.y) / scale)

        let constrainedRect = constrainBoundsRect(NSRect(origin: newOrigin, size: bounds.size))

        scroll(to: constrainedRect.origin)

        superview?.reflectScrolledClipView(self)
    }

    override func mouseUp(with _: NSEvent) {
        clickPoint = nil
    }
}