Show feedback in Variables pane while expanding values

[jmcphers]

System details:

Positron and OS details:

Positron Version: 2025.02.0 (Universal) build 134
Code - OSS Version: 1.96.0
Commit: 8b0487442af5034a1d2e5db80f0bf43e15db83e6
Date: 2025-01-29T18:30:03.745Z
Electron: 32.2.6
Chromium: 128.0.6613.186
Node.js: 20.18.1
V8: 12.8.374.38-electron.0
OS: Darwin arm64 24.2.0

Interpreter details:

Any

Describe the issue:

When you attempt to expand a variable in the Variables pane that takes a while to expand (e.g. because it's slow), there is no feedback that anything is happening. It just looks broken.

Steps to reproduce the issue:

(from #5869)

nc <- sf::read_sf(system.file("shape/nc.shp", package="sf"))
big_nc <- sf::st_sf(data.table::rbindlist(lapply(1:10000, \(x) nc)))

Run this code (after installing sf if needed) then expand the nc object (note that it works fine?), then the big_nc object.

Screen.Recording.2025-01-30.at.1.42.04.PM.mov

Nothing at all happens, though after 5 seconds you will eventually see an error popup.

Expected or desired behavior:

The user's gesture should be acknowledged, and the Variables pane should show that it's working on expanding the requested object (maybe by replacing the > with a spinner, or showing a progress stripe in the pane itself, etc.)

[jmcphers]

This also goes for attempts to open Data Explorers, which can also take a long time if the runtime is currently busy or it takes a long time to get the comm established due to collecting the initial shape, etc.

You can see some videos of user frustration with this in #4546.

[dfalbel]

FWIW the example code that could be used for reproducing will no longer be slow as it was fixed in posit-dev/ark#702. We can still reproduce it, by setting a long Sys.sleep() and then trying to interact with the variables pane.