Avoid unsharing stack if env refers top-level; ref
MODIFY
src/vm.c
src/vm.c
@@ -264,6 +264,7 @@ mrb_env_unshare(mrb_state *mrb, struct REnv *e)
mrb_value *p;
if (!MRB_ENV_STACK_SHARED_P(e)) return;
+ if (e->cioff == 0 && e->cxt.c == mrb->root_c) return;
MRB_ENV_UNSHARE_STACK(e);
if (!e->c) {
/* save block argument position (negated) */

Avoid unsharing stack if env refers top-level; ref #3643