.lesson-shell {
  grid-template-columns: 300px minmax(0, 1fr);
}

.lesson-nav,
.lesson-content,
.challenge-panel,
.plan-form {
  min-width: 0;
}

@media (max-width: 900px) {
  .lesson-shell {
    grid-template-columns: minmax(0, 1fr);
  }
}