Loading plot in separate JupyterLab tab collapses layout

[philippjfr]

As I originally outlined in #8084 since the layout PR you can get into situation where a layout collapses completely when reloading a notebook window. Since then I've further narrowed it down, and figured out that this only occurs in JupyterLab when the plot is in a different tab than the current one being displayed.

Here is a screencast demonstrating the issue, in it I have two notebooks open each with a bokeh layout. When I reload the notebook (by refreshing the page) the layout of widgets in the second notebook, which is not visible during the reload, collapses.

My guess is that this occurs because non-active JupyterLab tabs render but do not correctly report their size. I'm not really sure if this will be an easy thing to fix and it may have to happen in the JupyterLab extension telling bokeh to rerender the plots in the newly activated tabs.

[bryevdv]

cc @canavandl in case we need some Jlab extension help

[mattpap]

I need to reproduce this.

[mattpap]

The problem is similar to #8614. To make correct measurements, widget element must be displayed, as well as its each parent. This is not the case here. In case of tabs this can be easily fixed by using visibility property instead of display.

[mattpap]

Another issue is that notebook integration code loads CSS after JS, which makes stylesheets unavailable to layout when needed. This can be observed also in classical notebook.