Plotly mobile: Legend cannot be scrolled

[avsdev-cw]

Initially I thought this issue was the same as #2844, however I think it is different in one key point.

When displaying a plotly graph on a mobile device (particularly where the page has it's own scrollbar) and a large number of traces, the legend does not capture the press+drag event in the same way as the main plot. This means that the legend cannot be scrolled up or down on these devices.

Plotly.js version: v1.47.3

Example:
https://codepen.io/anon/pen/MRqOgE

[etpinard]

Thanks for reporting.

[etpinard]

Related: #2844

[andrewbaldock]

@etpinard Here's a PR you might consider for this issue: #3873