[Notebook] Infinitely growing visualisation panel for long filenames

[loichuder]

To reproduce:

1. Open in the notebook a file with a long name (>18 characters in my case but can be width-dependent)
2. Select an entry with a visualization (ex: root for NeXus file)
3. (Optional?) Try to reduce the width of the explorer panel: on mouse release, the panel will take its width back and the visualisation panel width will increase without bounds.

[axelboc]

Hmm, not sure how to fix this without touching at Jupyter Lab's styles, to replace the various display: table and float: left with flexbox... Maybe Jupyter Lab 3 brings changes to the notebook styles... 🤷

[loichuder]

silx-kit/h5web#780 is a good step forward: the h5web cell can still be wider than the notebook cell but it cannot grow indefinitely anymore.