scroll to top doesn't work correctly

**Describe the bug**
scroll to the top doesn't work correctly.

**To Reproduce**
create a tableView including too many items.
scroll to the middle of tableView and then call this:
```swift
if let navigationController = self.navigationController as? ScrollingNavigationController {
navigationController.showNavbar(animated: true, duration: 0.2, scrollToTop: true)
}
```
It scrolls to top but not actually to top of tableView.