Matching a clipped maximum and a clipped minimum

Integers $a$ and $b$ are each chosen independently and uniformly from $\{-10,-9,\dots,9,10\}$. What is the probability that $\max(0,a)=\min(0,b)$?

$121/441$. The left side is at least 0 and the right side at most 0, so both must equal 0. That happens exactly when $a\le 0$ and $b\ge 0$, each with 11 of the 21 values, giving $\left(\tfrac{11}{21}\right)^2$.