Gmail seems to have a flaw insofar as it acts as though tags apply only to the first email in a thread rather than to the whole thread (or to all emails in a thread).
This means that if I send an email, tag the thread "to-do" and then get a reply in the same thread, searching for
in:inbox label:to-do won't show the thread in the results, despite the fact that the thread is in my inbox and is tagged "to-do".
in:sent label:to-do will show the thread, but if I'm only interested in finding threads that are in my inbox (rather than archived threads, or messages that I've sent but which haven't yet received replies), then this is no help.
So, is it possible to search by thread rather than by email in Gmail?