Ansible handler not role-scoped

cmdline

$ ansible-playbook -i localhost, -v playbook.yml
No config file found; using defaults

PLAY [all] *********************************************************************

TASK [setup] *******************************************************************
ok: [localhost]

TASK [a : set_fact] ************************************************************
ok: [localhost] => {"ansible_facts": {"x": "foo"}, "changed": false}

TASK [a : debug] ***************************************************************
ok: [localhost] => {
    "msg": "a task here!"
}

TASK [b : set_fact] ************************************************************
ok: [localhost] => {"ansible_facts": {"x": "bar"}, "changed": false}

TASK [b : debug] ***************************************************************
ok: [localhost] => {
    "msg": "b task here!"
}

RUNNING HANDLER [a : dummy] ****************************************************
ok: [localhost] => {
    "msg": "a handler here! x=bar"
}

RUNNING HANDLER [b : dummy] ****************************************************
ok: [localhost] => {
    "msg": "b handler here! x=bar"
}

PLAY RECAP *********************************************************************
localhost                  : ok=7    changed=2    unreachable=0    failed=0


$ tree
.
├── playbook.yml
└── roles
    ├── a
    │   ├── handlers
    │   │   └── main.yml
    │   └── tasks
    │       └── main.yml
    └── b
        ├── handlers
        │   └── main.yml
        └── tasks
            └── main.yml

playbook.yml

- hosts: all
  roles:
    - a

roles_a_handlers_main.yml

- name: dummy
  listen: handler
  debug: msg="a handler here! x={{x}}"

roles_a_tasks_main.yml

- set_fact: x=foo

- debug: msg="a task here!"
  changed_when: true
  notify: handler

- include_role: name=b

roles_b_handlers_main.yml

- name: dummy
  listen: handler
  debug: msg="b handler here! x={{x}}"

roles_b_tasks_main.yml

- set_fact: x=bar

- debug: msg="b task here!"
  changed_when: true
  notify: handler