Add Viewer history navigation

[linem7]

Fixes #8647.

Viewer output now remains available for the lifetime of the current window instead of each generated HTML page replacing the previous one. The HTML and notebook-renderer toolbars include previous/next buttons for moving through the in-memory output list. URL previews keep their existing browser navigation behavior.

Release Notes

New Features

• Added previous and next navigation for Viewer output generated in the current project window.

Bug Fixes

• N/A

Validation Steps

1. Generate two HTML Viewer outputs, such as two reactable tables.
2. Confirm the newest output is shown.
3. Use the left and right buttons to move between both outputs.
4. Confirm Clear removes the output history.

Validated locally with:

• npm run compile-check-ts-native
• npx vitest run src/vs/workbench/contrib/positronPreview/test/electron-browser/positronPreviewService.vitest.ts

[github-actions]

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[linem7]

I have read the CLA Document and I hereby sign the CLA

[linem7]

recheck

[jmcphers]

This change overloads the arrows in a somewhat confusing way. Try this:

1. Create 3 HTML widgets
2. Press Back so that you're in the middle of the history. In this context, Back works to return the Viewer to a previous output.
3. Start a Shiny app (e.g. shiny::runExample("01_hello"))
4. Press Back again. Nothing happens.

This is because the arrows have changed meaning; with a URL open the Viewer now uses the arrows to navigate the content in the viewer, rather than between things you have loaded into the Viewer pane itself.

If you stop the Shiny app and make another widget, the arrows "work" again, but I think it's too confusing to overload them in this way.

[juliasilge]

Let us know if you are interested in taking another stab at this!