Verb
cross that bridge when one comes to it (third-person singular simple present crosses that bridge when one comes to it, present participle crossing that bridge when one comes to it, simple past and past participle crossed that bridge when one came to it)
(idiomatic) To deal with a problem or situation only if and when it arises.
It's possible we'll eventually have more books than available space for them, but we'll cross that bridge when we come to it.