karabiner elements で「英数キー押しっぱなし => Ctrl_L」「かなキー押しっぱなし => Alt_L」を実現する設定ファイル (~/.config/karabiner/assets/complex_modifications/ に置く)

personal_sobataro.json

{
  "title": "Change japanese_kana",
  "rules": [
    {
      "description": "Change japanese_eisuu to left_ctrl if pressed with other keys (Post japanese_eisuu when pressed alone)",
      "manipulators": [
        {
          "from": {
            "key_code": "japanese_eisuu",
            "modifiers": {
              "optional": [
                "any"
              ]
            }
          },
          "to": [
            {
              "key_code": "left_control"
            }
          ],
          "to_if_alone": [
            {
              "key_code": "japanese_eisuu"
            }
          ],
          "type": "basic"
        }
      ]
    },
    {
      "description": "Change japanese_kana to left_option if pressed with other keys (Post japanese_kana when pressed alone)",
      "manipulators": [
        {
          "from": {
            "key_code": "japanese_kana",
            "modifiers": {
              "optional": [
                "any"
              ]
            }
          },
          "to": [
            {
              "key_code": "left_option"
            }
          ],
          "to_if_alone": [
            {
              "key_code": "japanese_kana"
            }
          ],
          "type": "basic"
        }
      ]
    }
  ]
}