to reproduce:
  printf 'start \e[4mhi\e[5Cbye\e[0m end\n'
or a slightly more complicated one:
  printf 'start \e[4mhi\e[0m\e[5C\e[4mbye\e[0m end\n'

with hterm, we get:
  start <u>hi     bye</u> end

with other terminals we get:
  start <u>hi</u>     <u>bye</u> end

since these cells aren't populated, we shouldn't coalesce the "spaces" in between into the same span