The Loop That Stalls

f is monotone: true for every $x \le 9$ and false for every $x \ge 10$. This is meant to return the largest $x$ with f(x) true.

def last_true(lo, hi, f):
    while lo < hi:
        mid = (lo + hi) // 2
        if f(mid):
            lo = mid
        else:
            hi = mid - 1
    return lo

last_true(1, 10, f)

What happens?