Finding a todo
Search is the last of the views in the sidebar, below Summary. Use it when you know roughly what a todo said but not which list you put it in.
Press Ctrl+Shift+5 to jump straight there. The field is focused when you arrive, so you can start typing right away.
What it looks through
Everything you have, from every list: titles and the notes underneath them. So if you wrote "ring the bank" and put the account number in the notes, searching for the number finds it.
Finished todos are included. That's on purpose — the thing you can't find is very often something you already ticked off and half-forgot about. They show up struck through, exactly as they do everywhere else.
It forgives typos
You don't have to spell it right. Searching "dentst" finds "Dentist appointment", and "flightz" finds "Book flights to Sydney". Matches are ordered best-first, and a word in the title counts for more than the same word buried in the notes.
Searching starts once you've typed two characters. One letter would match almost everything, which isn't an answer.
Hidden lists stay hidden
If you've hidden a list, searching won't bring it back. That's deliberate: the filter is there so your personal lists aren't on screen when you're sharing it, and a search box that quietly undid that would be the worst possible place for it to leak.
Unhide the list and its todos are findable again straight away.
What it doesn't do
There are no extra filter buttons for due dates or priority. Today, Tomorrow and Next 7 days already answer "what's due", and the list filter already handles "which lists" — better than a search-only control would, since it tidies the sidebar too.
Your query isn't remembered. Come back tomorrow and the field is empty, because a search is a question you're asking now rather than a setting.