updatemenus: add <rect> background to scrollbox
updatemenus: add <rect> background to scrollbox

* Added a <rect> background to the scrollbox, so that events on the
  gaps between buttons aren't dropped.

iongroup/plotly.js,plotly/plotly.js,asolagmbh/plotly.js,asolagmbh/plotly.js,aburato/plotly.js,asolagmbh/plotly.js,etpinard/plotly.js,etpinard/plotly.js,plotly/plotly.js,plotly/plotly.js,etpinard/plotly.js,iongroup/plotly.js,iongroup/plotly.js,aburato/plotly.js,plotly/plotly.js