Ref T10163. When we think the user has finished typing a word (because they typed a space, period, or other similar characters) and nothing else they might type could possibly change the outcome (usually because the words they have typed already match nothing), just deactivate the autocomplete.

As a special case, if the word they have typed already select exactly one result, and they have already typed exactly that result, assume they just typed it from memory and deactivate.

• Typed @dog qwer zxcv and saw autocomplete deactivate on the space before z (on my local install, @dog is ambiguous but @dog qwer matches nothing).
• Typed @epriestley and saw autocomplete deactivate on space.