Add a big red stop button for Positron's Viewer pane

[juliasilge]

This PR addresses:

• Turn the "Run Shiny App" button into "Stop" once it's running #79 but only for Positron, so we probably shouldn't close it
• Update Shiny and Quarto extensions for new stop button affordance positron#10753 but only for Shiny (I still need to figure out Quarto)

This PR changes the preview behavior to keep track of the source terminal when in Positron, to instrument the new red stop button:

The red stop button should appear quite soon after you click the "play" button for either an R or Python Shiny app (it is not IMMEDIATE and I am wondering if we should try to hurry that up on the Positron side with an optimistic assumption that the app will start) and stay visible while the app is running.

• If you click the stop button while the app is running, the app should stop and then the Viewer pane should clear.
• If you do Ctrl+C in the terminal while the app is running, the app should also stop and the Viewer pane should also clear.

Just FYI, we have this functionality built in to Positron for other app types we support, like Streamlit and Dash and such. We need this change here to support the button for Shiny. Unfortunately, we don't have a lot of good options for testing here in the extension (no Positron test runner for extensions yet), but we can extend the Playwright tests we have in Positron once this change is in a released version of the extension. We do currently test this button for a Streamlit app and similar.

This does not effect any behavior when in VS Code.

[wch]

Looks good overall, but I think it would be better if we could get this to work in VS Code as well as Positron, so that Shiny users who use VS Code (or other forks) can also have a stop button. I realize this implementation uses some Positron-specific features, but I'll look into getting this to work in non-Positron as well.

[wch]

The as isn't necessary here.

Suggested change

	// eslint-disable-next-line @typescript-eslint/no-explicit-any
	const previewSourceType = (pst as any).PreviewSourceType;
	const previewSourceType = pst.PreviewSourceType;

[juliasilge]

Just as a heads up, this also applies to the Python interpreter.

[wch]

Thanks, I've updated this on main.

[wch]

I added some changes so that a stop button also appears in VS Code, which uses the Simple Browser for the app instead of Postron's Viewer panel.

When the app is running, a stop button appears in the Simple Browser when it is active:

And when the editor panel with the app file is active, it also shows a stop button there:

That latter case is supposed to also work in Positron, but I think there may be a bug in Positron related to displaying icons dynamically. In the package.json, there's the following:

      "editor/title/run": [
        {
          "command": "shiny.stopApp",
          "group": "navigation@-1",
          "when": "shiny.appRunning && (shiny.python.active || shiny.r.active)"
        }
      ],
      "editor/title": [
        {
          "command": "shiny.stopApp",
          "group": "navigation",
          "when": "shiny.appRunning && (shiny.python.active || shiny.r.active)"
        }
      ]

The editor/title/run entry controls the visibility of Stop Shiny App in dropdown menu for the Run button, and the editor/title entry controls the visibility of the button. In Positron, the first one works, but the second one does not seem to. Notice that Stop Shiny App is present below, but the Stop button is not:

It's the same logic for both, but only one of them shows up.

However, if I remove the when condition entirely (so that the button is always visible) the Stop button does appear initially, when the app isn't running, but then disappears after the app starts.

      "editor/title": [
        {
          "command": "shiny.stopApp",
          "group": "navigation"
        }
      ]

positron-shiny-stop-button-compressed.mp4