The most recent version 3.57 seems only supporting autocomplete with a general list of possible words instead of from the words that have been typed. It was asked in an early post TeXshop and autocompletion but the issue remains.
So I wonder if words that have recently been typed in Texshop will appear at the top of the autocomplete list, so that they can be selected easily. I have searched Texshop help but did not see it.

xyzzy(that shouldn't be a known word) and it appears in the list upon typingxyand hitting ESC. – egreg Nov 26 '15 at 22:23