At first I thought I was going to find that this was yet another situation where Apple "decides what we really want, even if we don't". But I found the answer to this. You can disable this behavior under Preferences - Text Editing - Editing and UNCHECK "Enable Typeover Completions". I couldn't find this previously because, for the life of me, I couldn't figure out what they meant by "Typeover Completions"! Also, several other options have tooltips... this one, with it's enigmatic description, doesn't have one.
Klethazeroth
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