extensions/markdown-language-features/server/src/workspace.ts/VsCodeClientWorkspace/openMarkdownDocument
class VsCodeClientWorkspace:	async openMarkdownDocument(resource: URI): Promise<md.ITextDocument | undefined> {
		const existing = this._documentCache.get(resource);
		if (existing) {
			return existing;
		}

		const matchingDocument = this.documents.get(resource.toString());
		if (matchingDocument) {
			let entry = this._documentCache.get(resource);
			if (entry) {
				entry.setInMemoryDoc(matchingDocument);
			} else {
				entry = new VsCodeDocument(resource.toString(), { inMemoryDoc: matchingDocument });
				this._documentCache.set(resource, entry);
			}

			return entry;
		}

		return this.openMarkdownDocumentFromFs(resource);
	}