TL;DR: "Culture or behavior" is currently off-topic, but it's hard to distinguish from Islam. Issues arise, e.g.: 1. declaring Muslim groups invalid by labelling their Islam as "culture"; 2. no means to distinguish which texts/authority determine Islam; 3. academics don't stop at these boundaries. Thus...
Question: Can we more precisely define which "culture or behavior" questions are off topic?
Currently we have the off-topic close reason:
Questions on Muslim culture or behaviour unrelated to the teachings of Islam are off-topic here. See: Islam SE: Not an Muslim Peer Support Group.
(For this post, let's put aside the distinction between "culture and behavior" and "peer support".)
Issue 1: What's Islam for someone is culture or behaviour for someone else.
Distinguishing the boundary between culture and Islam is tricky (see, e.g.: What distinguishes "cultural practices" from "applied Islam"?, particularly given the assumption on the on-topic page: For the purposes of this site, "Islam" includes all groups that identify themselves as Muslim.
If, say, a Muslim group considers female circumcision to be part of Islam, does that not make it part of Islam for the purposes of Islam.SE? If not, aren't we implicitly declaring the Muslim group invalid, and voting them off the island?
Issue 2: Whose texts and authority are valid for distinguishing "Islam" from "not Islam"?
G. Bach suggests "...valid foundation in authoritative texts" is how we distinguish culture from Islam. So whose texts are valid and who's an authority?
Issue 3: Academics don't stop at boundaries.
For example, here's two recent papers from the Journal of Islamic Studies:
The Ikhwān al-Ṣafāʼ on Religious Diversity, Janne Mattila (doi)
The aim of this study is to investigate, firstly, how the Ikhwān al-Ṣafāʼ explain the phenomenon of religious diversity, and, secondly, in what sense and to what extent they recognize other religions as valid.
Power vs. Authority: al-Juwaynī’s Intervention in Pragmatic Political Thought, Sohaira Siddiqui (doi)
This article explores Imām al-Juwaynī’s notion of power and authority as constructed through his text Ghiyāth al-Umam (Saviour of the Nation). ...
Going through such papers, there's many examples where Islam overlaps with culture and behavior (and history, politics, geography, psychology, etc.).
Proposed revision of the FAQ (on-topic) page, February 2017 (where I proposed rewording the boundary to "helps understand Islam")
Example questions raised on meta: