<rdar://problem/28409828> -[WKContentView(WKInteraction) canPerformAction:withSender:] returns NO for the Share action if the selected text is longer than 200 characters. A comment explains this by reference to the Define command (which no longer exists in iOS), but that seems incorrect. While long terms could never be looked up in the dictionary, the same restriction doesn’t exist for sharing, and indeed in both UIKit text views and in UIWebView, sharing is not restricted based on text.
Created attachment 290417 [details] Remove length limitation
Fixed in <https://trac.webkit.org/r206702>.