Regions don't get updated while you resize the browser window

* **C3 version**:
7.15
* **D3 version**:
5.15.1
* **Browser**:
Chrome Version 80.0.3987.163 , Safari 13.1
* **OS**:
OSX

If you resize your browser window, the chart gets nicely rescaled, however regions remain in the same place. Can be noticed here: http://c3js.org/samples/region_timeseries.html

This was reported in issue #992 in 2015 but appears to have resurfaced. It was noticed in October of last year by another reporter who commented but did not create a new issue.

Note I did try executing resize(,..) but that does not cause the region to be redrawn