updatemenus: fix positioning of scrollbars
updatemenus: fix positioning of scrollbars

* Do not assume that only one of the scrollbars is present.

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