`JupyterChart` accesses `.changed` property in callbacks

[azjps]

What happened?

In JupyterChart #3119, .changed I think is an implementation detail in Jupyter's backbone.js patch, and isn't supported in other AFM implementations such as marimo's:

This is using the JupyterChart example in the docs.

If I overwrite the anywidget ESM with

class AltairChart(altair.JupyterChart):
     _esm = """
            // ... rest of https://github.com/vega/altair/blob/5e3a0c7bcb2ebc74fbcaec47d4c9b6ec7828b1d9/altair/jupyter/js/index.js
            // for (const [param, value] of Object.entries(new_params.changed._params)) {
            for (const [param, value] of Object.entries(new_params)) {
     """

these errors go away and appears to work in both jupyter & marimo. I'm not sure if there's some other impact of this change though.

I came across this while trying to debug an AFM implementation in nicegui: zauberzeug/nicegui#5096 (reply in thread). Separately, I'm having trouble getting the binded JS interactions to work and don't have any leads, if someone would have advice on that?

What would you like to happen instead?

No console errors.

Which version of Altair are you using?

Tried with both altair=6 and altair=5.5.