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The list of word completion suggestions is purposely only shown when the cursor is positioned at the end of a word, or more specifically, when there is a whitespace character to the right of the cursor, including the end of line. Otherwise, you would be able to insert some word in the middle of another word, which has proven to be very confusing when writing a text.
Why
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does the app not
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seem to work directly after
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installation?
The Word Completion for Confluence app is deactivated by default. This is to avoid confusing your users who'd be suddenly confronted with the new word suggestion feature in page edit mode after the app has been installed. Administrators can choose to enable the feature globally or leave this decision to each individual user. The user manual describes how to do this in detail.