Add a setting to disable the dock panel padding

[jtpio]

References

Some users would prefer to have a more compact dock panel to save a few pixels here and there.

So this PR proposes to add a new setting to the JupyterLab shell to allow for such control on the dock panel.

Code changes

• Add a new --jp-dock-panel-padding CSS variable so themes can control the value directly, without adding it explicitly to the base themes
• Add a setting so the dock panel padding can be enabled or disabled by users

User-facing changes

The default stays the same as before.

jupyterlab-dockpanel-padding.mp4

Backwards-incompatible changes

None

AI usage

• YES: Some or all of the content of this PR was generated by AI.
• YES: The human author has carefully reviewed this PR and run this code (keep this PR "draft" until the answer is YES)
• AI tools and models used: Opus 4.6 & GPT 5.4

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[krassowski]

I like the idea. I would use this setting in a few places myself.

It could be more discoverable if it was added in Theme settings but I see how it belongs in Shell given that this element is only used by JupyterLab Shell.

I also wondered if we should call it --jp-private-dock-panel-padding to avoid confusing theme authors who may want to set it to to a different value say 3px, but I think if they set it on :root then it will still work as expected.

[jtpio]

Thanks for the review!

I also wondered if we should call it --jp-private-dock-panel-padding to avoid confusing theme authors who may want to set it to to a different value say 3px, but I think if they set it on :root then it will still work as expected.

That makes sense, just pushed a new commit to do that rename.