src/vs/workbench/browser/parts/editor/editorsObserver.ts/EditorsObserver/addMostRecentEditor
class EditorsObserver:	private addMostRecentEditor(group: IEditorGroup, editor: EditorInput, isActive: boolean, isNew: boolean): void {
		const key = this.ensureKey(group, editor);
		const mostRecentEditor = this.mostRecentEditorsMap.first;

		// Active or first entry: add to end of map
		if (isActive || !mostRecentEditor) {
			this.mostRecentEditorsMap.set(key, key, mostRecentEditor ? Touch.AsOld /* make first */ : undefined);
		}

		// Otherwise: insert before most recent
		else {
			// we have most recent editors. as such we
			// put this newly opened editor right before
			// the current most recent one because it cannot
			// be the most recently active one unless
			// it becomes active. but it is still more
			// active then any other editor in the list.
			this.mostRecentEditorsMap.set(key, key, Touch.AsOld /* make first */);
			this.mostRecentEditorsMap.set(mostRecentEditor, mostRecentEditor, Touch.AsOld /* make first */);
		}

		// Update in resource map if this is a new editor
		if (isNew) {
			this.updateEditorResourcesMap(editor, true);
		}

		// Event
		this._onDidMostRecentlyActiveEditorsChange.fire();
	}