Allow to store/resume open editors across Continue On transitions

[joyceerhl]

Split out from #183449

[bpasero]

For editors, we use a Memento to store and restore editor state. We currently use StorageTarget.MACHINE and would have to change that to StorageTarget.USER:

vscode/src/vs/workbench/browser/parts/editor/editorPart.ts

Line 138 in b21537c

private readonly workspaceMemento = this.getMemento(StorageScope.WORKSPACE, StorageTarget.MACHINE);

Mementos are just simple wrappers around the storage service that make it a bit easier to work with.

The state of editors begins to be created here:

vscode/src/vs/workbench/browser/parts/editor/editorPart.ts

Lines 1315 to 1319 in b21537c

	const uiState: IEditorPartUIState = {
	serializedGrid: this.gridWidget.serialize(),
	activeGroup: this._activeGroup.id,
	mostRecentActiveGroups: this.mostRecentActiveGroups
	};

Serialising the grid translates to a call to toJSON for each node:

vscode/src/vs/workbench/browser/parts/editor/editorGroupView.ts

Lines 2013 to 2015 in b21537c

	toJSON(): ISerializedEditorGroupModel {
	return this.model.serialize();
	}

Each editor of a editor group has its own serialiser that is called:

vscode/src/vs/workbench/common/editor/editorGroupModel.ts

Line 1036 in b21537c

const value = editorSerializer.serialize(editor);

I count around 20 serialisers that each own how the data is serialised.

The one for files has this shape:

vscode/src/vs/workbench/contrib/files/browser/editors/fileEditorHandler.ts

Lines 38 to 45 in b21537c

	const serializedFileEditorInput: ISerializedFileEditorInput = {
	resourceJSON: resource.toJSON(),
	preferredResourceJSON: isEqual(resource, preferredResource) ? undefined : preferredResource, // only storing preferredResource if it differs from the resource
	name: fileEditorInput.getPreferredName(),
	description: fileEditorInput.getPreferredDescription(),
	encoding: fileEditorInput.getEncoding(),
	modeId: fileEditorInput.getPreferredLanguageId() // only using the preferred user associated language here if available to not store redundant data
	};

@joyceerhl in this case where resource.toJSON() is used, how would we support continue on, given its not the URI verbatim but already the JSON form?

[bpasero]

Actually, this may be fine, URI.toJSON puts the $mid marker that I think we use:

vscode/src/vs/base/common/uri.ts

Lines 472 to 474 in b21537c

	const res = <UriState>{
	$mid: MarshalledId.Uri
	};

[alexandruavadanii]

Hi,
Thank you for working on yet another nice feature!

I wonder whether this will also cover custom URI schemes handled by TextDocumentContentProvider. While not being a major problem, I always saw the read-only editors being silently discarded, even without using the "continue on" functionality and just reloading the current workspace.

If that's out of scope here, I can create a separate issue and try to better describe my use-case, maybe even provide a simple demo for testing.

[bpasero]

In my testing, I can see how editors restore but with the original URI and not the transformed one. This means that somehow the URI transformation does not take place.

@joyceerhl can you continue to investigate.