My Day 10 solution is here:
https://github.com/jorispz/aoc-2018/blob/master/src/commonMain/kotlin/P10.kt
Simulating particles we have seen in earlier years, and as before the problem is in defining a stopping condition. I cheated somewhat and resorted to thoughtful trial-and-error, but I am sure some people here or on Reddit will come up with cleverer stuff.
Today's timings for both parts (first/best of 25): JVM
262/44
, JS
454/254
, native
5830/5830
(first run was the best)