bpmn-io/bpmn-js

Alignment tools for labels don't work inside of participant/lane

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#2,371 opened on Dec 22, 2025

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buggood first issuemodelingspring cleaning

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Description

Describe the Bug

Alignment helpers (snap lines) for external labels show outside of participant, but not within participant / lane:

Image

Steps to Reproduce

  • Align external label with Gateway/Event outside of participant
    • See that label target is "peer" used for horizontal center alignment
  • Align external label with Gateway/Event inside of participant
    • See that label target is not accounted as "peer", and no assistance is given

Can be reproduced on demo.bpmn.io.

Expected Behavior

Same behavior, independent of the modeling context.

Environment

  • Browser: Chrome 148
  • OS: Any (Linux, Ubuntu 24)
  • Library version: v18.9.1 / demo.bpmn.io

Contributor guide

Research direction
Examine the alignment tool implementation in bpmn js to understand how peers are detected for snap lines. Compare behavior inside and outside a participant/lane. Look for differences in the element hierarchy or coordinate system that cause peers to be missed inside participants.
Tech stack
javascript
Domain
frontend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
BPMN knowledgeJavaScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70