Workaround for http://openradar.appspot.com/46395110

UndoManagerWorkaround.swift

extension UndoManager {
    @objc func workaround_pspdf_endUndoGrouping() {
        guard self.groupingLevel > 0 else { return }
        self.workaround_pspdf_endUndoGrouping()
    }

    // Ensure this is only called once!
    fileprivate static func InstallWorkaroundForRdar46395110() {
        let originalSelector = #selector(UndoManager.endUndoGrouping)
        let swizzledSelector = #selector(UndoManager.workaround_pspdf_endUndoGrouping)
        let undoManagerClass = UndoManager.self
        let originalMethod = class_getInstanceMethod(undoManagerClass, originalSelector)!
        let swizzledMethod = class_getInstanceMethod(undoManagerClass, swizzledSelector)!

        let didAddMethod = class_addMethod(undoManagerClass, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod))
        if didAddMethod {
            class_replaceMethod(undoManagerClass, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod))
        } else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    }
}