Post 2.0 improvements to LabIcon: work with all svg loaders, improve performance, fix issue with menus from extensions

[telamonian]

References

jupyter/nbdime#512

Some improvements to LabIcon.

@vidartf With these changes you should be able to use LabIcon/@jupyterlab/ui-components in just about any project regardless of how svg loading is configured.

Code changes

• LabIcon should now work with all commonly-used svg loaders/loading schemes

  • previously, LabIcon expected svgs to be loaded using a raw loader (such as Webpack's raw-loader) that directly loaded the raw data from an .svg file as a string
  • accomplished by shimming the input svgstr

• 15-20x speedup of icon creation (on the javascript side)

  • accomplished by caching the intermediates produced while parsing LabIcon.svgstr into actual svg nodes
  • results in an overall 2x speedup of icon creation + render
  • profiling of creating/rendering ~1500 filebrowser items

    • old code:

      

    • new code (compare cloneNode to resolveSvg above):

      

• Fix icons in menus added to main menubar by extensions, and ensure that correct renderer is used for all submenus

  • done via fixes to the implementation of MenuSvg

User-facing changes

• improved performance related to icons (especially when opening dirs with 1000+ files in the filebrowser)

• icons should now work correctly in menus added to main menubar and in all submenus (including context submenus)

Backwards-incompatible changes

None

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[telamonian]

This PR is now ready for review.

I already went through and checked everything, but ideally whoever reviews this will build it locally and double check that there's no regressions in the menu icons.

[saulshanabrook]

The summary from this meeting that for 3.0 we should fix some of these things upstream in lumino, but for now we can fix them here without breaking changes.