src/vs/workbench/browser/parts/editor/editorActions.ts/OpenPreviousEditor/navigate
class OpenPreviousEditor:	protected navigate(): IEditorIdentifier | undefined {

		// Navigate in active group if possible
		const activeGroup = this.editorGroupService.activeGroup;
		const activeGroupEditors = activeGroup.getEditors(EditorsOrder.SEQUENTIAL);
		const activeEditorIndex = activeGroup.activeEditor ? activeGroupEditors.indexOf(activeGroup.activeEditor) : -1;
		if (activeEditorIndex > 0) {
			return { editor: activeGroupEditors[activeEditorIndex - 1], groupId: activeGroup.id };
		}

		// Otherwise try in previous group
		const previousGroup = this.editorGroupService.findGroup({ location: GroupLocation.PREVIOUS }, this.editorGroupService.activeGroup, true);
		if (previousGroup) {
			const previousGroupEditors = previousGroup.getEditors(EditorsOrder.SEQUENTIAL);
			return { editor: previousGroupEditors[previousGroupEditors.length - 1], groupId: previousGroup.id };
		}

		return undefined;
	}