ContentOffset calculated wrong when pasting long text into textview

[intoxicated]

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.

Description

I'm experiencing unexpected behavior of textview when pasting long text into textview.

Reproducible in:

• [x] This is reproducible in the sample project. SlackTextViewController version: iOS version(s): 10.2 Device(s): iphone 7 plus

Steps to reproduce:

1. create a string for auto completion (i.e /tellme -> replace with "very long text")
2. click on auto completion item

Expected result:

expected to see text and cursor at the end of text

Actual result:

blank textview (highly possible to mis-calculated contentOffset of textview) but text appeared after scrolled textview and adjusted contentOffset properly