Chart.js legend callbacks: unresolved dataset visibility toggle timing

Goal

Determine how the Chart.js legend onClick handler updates the dataset.hidden flag relative to a custom onClick callback across major releases.

Constraints

Chart.js 2.x updates the flag before the callback, while 3.x and 4.x update it after. The library documentation lists the callback signature but omits the order of operations, leaving developers uncertain when to read the flag within custom callbacks.

Uncertainty

Because the toggle order affects the state seen by custom legend logic, it can lead to bugs when multiple datasets are programmatically toggled or when the callback modifies visibility. No official hook exists to expose the sequence, and future releases may change it.

Questions

• In Chart.js 4.x, does dataset.hidden update after the onClick callback or before?
• Should custom legend callbacks rely on the pre‑toggle or post‑toggle state of dataset.hidden?
• Will future Chart.js releases provide a dedicated hook or documented order for the visibility toggle?