//
// Editor area
//

@edit-area-border-color: @border-color;

.tox {
  .tox-edit-area {
    border: 1px solid @edit-area-border-color;
    display: flex;
    flex: 1;
  }

  .tox-edit-area__iframe {
    border: 0;
    box-sizing: border-box;
    flex: 1;
    width: calc(100% - 2px); //deduce border width
  }
}

//
// Editor area
//

@edit-area-border-color: @border-color;

.tox {
  .tox-edit-area {
    border: 1px solid @edit-area-border-color;
    display: flex;
    flex: 1;
  }

  .tox-edit-area__iframe {
    border: 0;
    box-sizing: border-box;
    flex: 1;
    width: calc(~"100% - 2px"); //deduce border width
  }
}

Fix width on edit area
Fix width on edit area
