Why placeholder text jump upward little when first tap searchBar？

I put a searchBar in first cell in the UITableView (not in UITableView header!) :

firstCell.contentView.addSubview(searchController.searchBar)


and I canceled hide navigation bar when search happened :

searchController.hidesNavigationBarDuringPresentation = false


and full setup searchController code in below :

func setupSearchController(){

//custom Search Results Controller
let folderSearchVC = FolderSearchViewController()
searchController = UISearchController(searchResultsController: folderSearchVC)

//cancel hide NavigationBar
searchController.hidesNavigationBarDuringPresentation = false

searchController.searchResultsUpdater = folderSearchVC
searchBar = searchController.searchBar

searchBar.placeholder = "搜索习惯组"

firstCell.contentView.addSubview(searchBar)
}


But when I first activate the searchBar,the placeholder text will jump upward a little,only for the first time (demo in below GIF):

You can see in the demo gif ,when I first tap searchBar,the placeholder text jump upward a little,and follow tap is all OK！

my Q is how to forbid placeholder text to jump upward ???
thanks ;)

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