LibreOffice's Word Completion, its defaults included: words from eight
letters up are collected as they are finished, and the first three
letters offer the rest. The offer is a widget decoration after the caret,
so it is outside the document until Enter takes it — nothing to undo when
it is not taken.
Only while typing: a bare caret move drops the offer, or an Enter meant to
split the paragraph would complete a word instead. The settings share the
AutoCorrect dialog, as they share LibreOffice's.