At resonance the displacement lags the drive by a quarter cycle

Evidence

As for the phase, tan δ blows up when f = f0. meaning that the denominator cos δ = 0 there, which happens when δ = π/2. If the motion is given by y(t) (equation 10.2), then the velocity υf due to the sinusoidal driving at frequency f is When δ = π/2, υf(t) = 2πf A sin(2π ft), which means velocity and force are in phase; they are both sines with no phase shift. — Eric J. Heller, Why You Hear What You Hear- An…, 980