Enable strict CSS containment for MainAreaWidget

[krassowski]

References

Relates to jupyterlab/lumino#504. Supersedes #13672.

This is a smaller version of jupyterlab/lumino#506 which I hope can be backported and included in 3.x.

This PR is not needed on 4.x once jupyterlab/lumino#506 is merged and released.

Code changes

Addition of:

.jp-MainAreaWidget {
  contain: strict;
}

User-facing changes

• Notebook outputs producing html elements with transform: translate* styles will not be drawn outside of the notebook boundaries.
• Substantial performance boost (reduction in layout cost) on Chromium browsers

Backwards-incompatible changes

As above.

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[fcollonval]

Thanks @krassowski