[WebPDF] How to better detect that notebook & widgets have finished rendering

[SylvainCorlay]

I had to tag a patch release (6.0.5) reverting the move to networkidle2, which prevented custom widgets to render when doing --to webpdf --execute because of the CDN requests.

However, this change causes the scotch_dashboard.ipynb to timeout in webpdf. Just adding page.waitfor(100) after the networkidle2 makes it work... So we should probably work on a better signal that the widgets have finished rendering.

cc @maartenbreddels

[mgeier]

I'm not sure, but this might be to some degree related to #913.

[maartenbreddels]

I added a global promise window.widgetManagerPromise in this PR https://github.com/voila-dashboards/voila/pull/574/files so that we can wait for the manager to first build the widgets. But even then it can mean a widget is still doing work. For instance in #901 I hacked it by deep inspection of the widget, and wait for leaflet's layer.obj.on('load', ..) to trigger. I think this basically asks for an API from the widget manager, and a way to get access to all widget managers.