Calegari's spectral-gap conjecture for stable commutator length in free groups

Let $c$ be an integral chain in a free group, and consider admissible surfaces representing $c$. The stable commutator length $\operatorname{scl}(c)$ is the associated stable commutator length.

Calegari's conjecture. For any integral chain $c$ in a free group,

$$\operatorname{scl}(c)\geq\frac{1}{2}$$

unless an admissible surface of $c$ is a union of annuli, in which case $\operatorname{scl}(c)=0$.

This conjecture asserts a spectral gap for stable commutator length in free groups: outside the annular case, stable commutator length is bounded below by $1/2$. The supplied text does not provide evidence of a resolution.