handleExtraCanvasScroll(ev) {

    // Solves: https://github.com/felixhayashi/TW5-TiddlyMap/issues/409
    if (this.isInSidebar) {
      ev.preventDefault();
    }
  }
This handles the extraneous event fired by Firefox whenever a
DOMMouseScroll event occurs. We just want to swallow it.
Solves: https://github.com/felixhayashi/TW5-TiddlyMap/issues/306