src/vs/workbench/browser/parts/editor/editorActions.ts/OpenNextEditor/navigate
class OpenNextEditor:	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 + 1 < activeGroupEditors.length) {
			return { editor: activeGroupEditors[activeEditorIndex + 1], groupId: activeGroup.id };
		}

		// Otherwise try in next group
		const nextGroup = this.editorGroupService.findGroup({ location: GroupLocation.NEXT }, this.editorGroupService.activeGroup, true);
		if (nextGroup) {
			const previousGroupEditors = nextGroup.getEditors(EditorsOrder.SEQUENTIAL);
			return { editor: previousGroupEditors[0], groupId: nextGroup.id };
		}

		return undefined;
	}