Scrollbar UI for virtually rendered notebook

[afshin]

This PR updates the WindowedList class that the notebook widget inherits from to have customizable renderers (which allows for adding aria labels to make windowed lists more accessible and allows for customization of the UI).

The renderer in turn is used to add a scrollbar component to replace the native scrollbar that jumps when virtual notebook rendering is enabled.

The scrollbar (and its toolbar button) only appear when "Windowing mode" is set to "full".

References

• supersedes Add a renderer for windowed lists #15036
• fixes Scrollbar jumps back in a virtualized notebook on scrolling #13343

Code changes

• Updates the WindowedList API to allow for custom renderers of windowed list subcomponents
• Adds a virtual scrollbar UI to replace the native scrollbar when virtual rendering is turned on
• Updates the command toolbar button to respect isEnabled, isVisible, isToggled.

User-facing changes

Adds an optional new scrollbar control for virtually rendered notebooks.

before

virtual-scrollbar-before.mov

after

virtual-scrollbar-after.mov

Backwards-incompatible changes

N/A

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[krassowski]

This is kind of hard to manage due to faults in GitHub UI. After every force-push I have to click 7 times to get the most recent discussion show up:

I also get two emails, one for the pull request and one for the commit with "fix plugin manager test @krassowski" message.

As a partial alleviation, can I kindly suggest to:

• squash all the Update Playwright Snapshots commits as these do not provide any thing useful (in fact I would say lets revert all snapshots and correct the CSS styles so that there are no changes at all; something I started working in the evening but did not manage to get in time)
• (minor) remove usernames from commit messages
• remove all comments requesting bot snapshot updates, and stop issuing those. I think we do not need any single snapshot to change in this PR, it just makes the review harder.

[krassowski]

I see how this could be merged soon if the style changes are limited to when the scrollbar is indeed active. It would ensure no breakages downstream. This could be confirmed by snapshots having no changes.

[krassowski]

A few tests are failing not because of snapshots but because this class name in test file needs updating:

--- a/galata/test/jupyterlab/windowed-notebook.test.ts
+++ b/galata/test/jupyterlab/windowed-notebook.test.ts
@@ -40,7 +40,7 @@ async function getInnerHeight(panel: ElementHandle<Element>) {
 async function getWindowHeight(panel: ElementHandle<Element>) {
   return parseInt(
     await panel.$eval(
-      '.jp-WindowedPanel-window',
+      '.jp-WindowedPanel-viewport',
       node => (node as HTMLElement).style.minHeight
     ),
     10

Is this change documented in the migration guide? I wonder if this could impact downstream (themes). A safe path would be to keep both and deprecate -window variant, but possibly mentioning this in the migration guide could be enough.