Check logic around changing kernels in JupyterLab manager

[jasongrout]

In 9d999d7 (part of #2532), we introduced changes refactoring how kernels are used in the JupyterLab manager.

Looking over the code in https://github.com/jupyter-widgets/ipywidgets/blob/master/packages/jupyterlab-manager/src/manager.ts, I think it would be good to check a few things:

• When a session context changes a kernel, is this._kernel reset?
• In ipywidgets 7.x, we waited for the context to be ready in _loadFromKernel - is that still done in the refactor, or is it needed?
• Consider this comment again: Labmanager refactor #2532 (comment)
• Consider this comment again: Labmanager refactor #2532 (comment)

[jtpio]

When a session context changes a kernel, is this._kernel reset?

Does this question only apply to the KernelWidgetManager?

ipywidgets/jupyterlab_widgets/src/manager.ts

Line 409 in 7411536

export class KernelWidgetManager extends LabWidgetManager {

The WidgetManager (used for notebooks) seems to be handling a SessionContext, which should handle kernel restarts.

ipywidgets/jupyterlab_widgets/src/manager.ts

Lines 494 to 504 in 7411536

	context.sessionContext.kernelChanged.connect((sender, args) => {
	this._handleKernelChanged(args);
	});
	context.sessionContext.statusChanged.connect((sender, args) => {
	this._handleKernelStatusChange(args);
	});
	context.sessionContext.connectionStatusChanged.connect((sender, args) => {
	this._handleKernelConnectionStatusChange(args);
	});

So accessing this.kernel should return the current kernel for the session context:

ipywidgets/jupyterlab_widgets/src/manager.ts

Lines 619 to 621 in 7411536

	get kernel(): Kernel.IKernelConnection | null {
	return this._context.sessionContext?.session?.kernel ?? null;
	}

[jtpio]

In ipywidgets 7.x, we waited for the context to be ready in _loadFromKernel - is that still done in the refactor, or is it needed?

Looking at the code, _loadFromKernel seems to be called from the WidgetManager here:

ipywidgets/jupyterlab_widgets/src/manager.ts

Line 565 in 7411536

await this._loadFromKernel();

And from KernelWidgetManager here:

ipywidgets/jupyterlab_widgets/src/manager.ts

Line 454 in 7411536

await this._loadFromKernel();

With _loadFromKernel defined in the base LabWidgetManager class:

ipywidgets/jupyterlab_widgets/src/manager.ts

Line 99 in 7411536

protected async _loadFromKernel(): Promise<void> {