Override callbacks are not getting cleaned up when a callback is cleared

[jasongrout]

notebook/notebook/static/services/kernels/kernel.js

Line 894 in 92354d5

Kernel.prototype.clear_callbacks_for_msg = function (msg_id) {

is not disposing any overrides registered for a given message id, so we are still finding the override callbacks at

notebook/notebook/static/services/kernels/kernel.js

Line 882 in 92354d5

var overrides = this._msg_callbacks_overrides[msg_id];

even if the original callbacks are gone.

We should delete the override array and remove every callback in it from the callbacks array.

(See jupyter/jupyter_client#297 (comment) for more context)

[jasongrout]

I think this will mean that Output widgets won't work as well with threaded output - since the original callback will be cleared when the main thread sends back the idle status, the override will be cleared then and the Output widget won't get the output.

[takluyver]

@jasongrout did you want to make a PR for this?

[takluyver]

Bumped again, but we can still un-bump it if you want to work on it for 5.5.