fix(class): place cardinality labels beside their ends with the ELK layout

[ashishjain0512]

Summary

Resolves #8329

With the ELK layout (the default on develop), class diagram cardinality labels were clipped, hidden behind class boxes, and not placed at their relation's end. ELK was never told those labels existed, so it reserved no room for them. The painter then placed each label 35px along the path plus a 15px sideways offset, which landed past the first bend and under the neighbouring box (e.g. many at x 159–195 with Duck starting at x 183).

• Measure: insertEdgeLabel records each terminal label's measured size and centre offset on the edge (edge.terminalLabelSizes).
• Lay out: the ELK layered algorithm now receives those labels as TAIL/HEAD labels, padded by half a marker so they clear the arrowheads. It reserves space beside the port and places them. The placed centres are read back into edge.terminalLabelCenters.
• Follow the endpoint: when the clipping or straightening passes slide an endpoint along the node side after ELK, its labels move with it.
• Paint: the four copy-pasted terminal-label blocks in the common painter are now one terminalLabelTranslate(). It uses the layout's centre when there is one and otherwise the existing calcTerminalLabelPosition, so dagre and the other layouts render exactly as before.
• Sides: ELK puts every end label on one fixed side, so start and end cardinalities shared a side. Each label is now mirrored onto dagre's side (start labels right of the direction of travel, end labels left), but only when that space is free of edges, class boxes and other labels; otherwise it keeps the side ELK reserved.
• Clipping (dagre too): setTerminalWidth hard-coded the cardinality box to 12px high, cutting the bottom ~2px of every HTML-label cardinality (0..1 read as ∩ 1 at larger sizes). The box is now sized from the measured label. This and the end-label centring are the changes that affect dagre.
• Centred end labels (dagre too): end cardinalities were added beside their centring group instead of inside it, so they were drawn from their top-left corner, with a fixed -5, -5 nudge in calcTerminalLabelPosition standing in for centring. Long end labels such as many ran past their intended spot and under the class box in dagre. They are now centred like start labels, and the nudge is gone.
• Guards: only ELK layered places end labels. Other ELK algorithms (mrtree, force, …) keep the previous placement, and so do labels left unplaced at a container's origin.

Classification

• Change type: shared rendering-util (ELK layout, common painter, edge label measurement)
• Breaking change: no. The new Edge fields and types are additive and optional. Class diagram output changes under ELK by design; that is the fix.
• Shared code touched: yes: rendering-util/types.ts, rendering-elements/edges.js, layout-algorithms/common/index.ts, layout-algorithms/elk/render.ts. Only class diagrams produce terminal labels. Only ELK layered uses the new placement; the clipping fix and the end-label centring in edges.js / utils.ts also change dagre class diagrams with cardinalities.

Verification

• TDD: elk/__tests__/terminalLabels.spec.ts runs a real ELK layout of the issue's example. It asserts every cardinality sits beside its own end, clear of its marker and of every class box. It fails on develop with the issue's symptoms and passes with the fix. The regression tests for mrtree/force fail when the algorithm guard is removed.
• Lint: passed (0 errors, no new warnings in source files)
• Unit tests: passed (vitest run packages/mermaid/src/rendering-util packages/mermaid/src/diagrams/class packages/mermaid-layout-elk, 1265 tests)
• E2E snapshot tests: added ELK-4 (the issue's TB and LR diagrams), ELK-5 (SVG labels), ELK-6 (namespaces), ELK-7/ELK-8 (a proportional font with ELK and dagre, where the old 12px box clipped) and ELK-9 (dagre LR, where many used to run under Duck) in e2e/rendering/class/classDiagram-elk-v3.spec.js
• Visual spot-check: verified in the browser (TB, LR, SVG labels, namespaces)
• Local before/after pixel comparison against develop over 329 class-related e2e screenshots:
  • Changed: the class diagrams with cardinalities (ELK and dagre) and the new tests.
  • Byte-identical: every other diagram type. ELK renders were unchanged by the end-label centring, which confirms the ELK path already compensated for the old anchoring.
• Unit tests also cover dagre's sides (TB and LR), the free-space check with a neighbouring port (sizes measured in the browser), setTerminalWidth never sizing below the measured label, end labels being centred like start labels, and calcTerminalLabelPosition placing end labels as the mirror image of start labels.
• Full e2e: left to CI/Argos. Locally I ran the class specs and .mmd fixtures shown above.
• Changeset: generated (patch, mermaid and @mermaid-js/layout-elk)

Known follow-up

#8335 (a cardinality crossing the edge from an adjacent port): its example no longer reproduces. With the free-space check, the label stays on the side ELK reserved when dagre's side is taken, and no cardinality crosses an edge in any of the test diagrams. I'll leave #8335 open until this merges, then confirm it there.

Dagre end labels that sat a few pixels under the class box (e.g. many against Duck) are fixed here by the centring.

Summary

• Fixes cardinality-label placement in ELK layered class diagrams. The renderer measures terminal labels and passes their dimensions to ELK. The painter uses ELK’s label centers when available.
• Keeps labels aligned with endpoints that move during later layout passes. The renderer mirrors labels to the requested side when that position is clear of nodes, group frames, edge routes, and other labels.
• Sizes terminal-label boxes from measured text dimensions to reduce clipping. This also affects dagre class diagrams with cardinality labels.
• Leaves terminal-label placement unchanged for non-layered ELK algorithms and clears stale label centers between layouts.
• Centers end labels consistently with start labels.

Tests

• Adds unit tests for ELK terminal-label placement, collisions, moved endpoints, repeated layouts, and measured label sizing.
• Adds end-to-end snapshot tests for ELK and dagre class diagrams, including HTML and SVG labels, namespaces, and proportional fonts.
• The PR description reports that lint and unit tests passed. Full end-to-end testing was left to CI.

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🦋 Changeset detected

Latest commit: 776e687

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

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@mermaid-js/layout-elk	Patch

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📒 Files selected for processing (10)

• .changeset/elk-class-cardinality-labels.md
• docs/config/setup/mermaid/interfaces/LayoutData.md
• e2e/rendering/class/classDiagram-elk-v3.spec.js
• packages/mermaid/src/rendering-util/layout-algorithms/common/index.ts
• packages/mermaid/src/rendering-util/layout-algorithms/elk/__tests__/terminalLabels.spec.ts
• packages/mermaid/src/rendering-util/rendering-elements/edges.js
• packages/mermaid/src/rendering-util/rendering-elements/edges.terminalLabels.spec.js
• packages/mermaid/src/rendering-util/types.ts
• packages/mermaid/src/utils.spec.ts
• packages/mermaid/src/utils.ts

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• .changeset/elk-class-cardinality-labels.md
• docs/config/setup/mermaid/interfaces/LayoutData.md

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📝 Walkthrough

Walkthrough

The change records terminal-label dimensions and centers, passes measured labels to ELK layered layout, and uses returned positions when rendering class-diagram cardinality labels. Tests cover label placement, endpoint movement, and several ELK class-diagram configurations.

Changes

ELK Cardinality Labels

Layer / File(s)	Summary
Terminal-label measurements packages/mermaid/src/rendering-util/types.ts, packages/mermaid/src/rendering-util/rendering-elements/edges.js, packages/mermaid/src/rendering-util/rendering-elements/edges.spec.js, packages/mermaid/src/rendering-util/rendering-elements/edges.terminalLabels.spec.js	Edge gains maps for terminal-label sizes and centers. Edge rendering records measured label dimensions and sizes HTML labels from measurements. Tests check the measured box dimensions and the label group structure.
ELK terminal-label placement packages/mermaid/src/rendering-util/layout-algorithms/elk/render.ts, packages/mermaid/src/rendering-util/layout-algorithms/elk/__tests__/terminalLabels.spec.ts	ELK layered layout receives measured terminal labels and returns label centers. The renderer clears stale centers, adjusts them when endpoints move, and checks candidate placements against diagram geometry. Tests check label distance, overlap, non-layered layouts, and endpoint movement.
Shared positioning and rendering coverage packages/mermaid/src/rendering-util/layout-algorithms/common/index.ts, packages/mermaid/src/utils.ts, packages/mermaid/src/utils.spec.ts, e2e/rendering/class/classDiagram-elk-v3.spec.js, .changeset/elk-class-cardinality-labels.md, docs/config/setup/mermaid/interfaces/LayoutData.md	The shared renderer uses stored centers and fallback positioning. Utility tests cover end-label coordinates. Snapshot tests cover HTML and SVG labels, namespaces, and ELK and Dagre layouts. The changeset records patch releases, and the documentation source links are updated.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Suggested reviewers: knsv

Merge Risk: 🟡 Moderate · up to 776e6

ELK may reserve less space than an HTML cardinality label’s viewport occupies. Confirm that this does not cause placement conflicts before merging.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 776e6

The change affects label placement across shared rendering paths. The inspected flow does not change access to data or privileges, but its full security coverage is not established.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The inspected effects extend through shared layout and painting code, but the identified sink is visual label positioning rather than access to a tenant, credential, or data store.

Trust Boundaries and Controls

• observed — Successful ELK edge application clears stored terminal-label centers before recording new placements; the shared painter uses its path-based fallback when no center is present.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 10 files. (2 skipped:…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: fixing cardinality-label placement beside relation ends for the ELK layout.
Description check	✅ Passed	The description is complete and relevant. It includes the issue reference, implementation details, design decisions, verification results, test coverage, changeset status, and known follow-up work. Th…
Linked Issues check	✅ Passed	The PR addresses [#8329]. ELK layered layout measures terminal labels, sends their sizes to ELK, records placed centers, and follows moved endpoints. The painter uses placed centers and checks collisi…
Out of Scope Changes check	✅ Passed	The changes stay within the cardinality-label placement and sizing scope of [#8329]. Shared translation logic, terminal-label types, measured-label data, ELK layout logic, dagre sizing and centering, …

Full details: Docstring Coverage

Explanation

Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 10 files. (2 skipped: 2 unsupported.)

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commit: 776e687

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Codecov Report

❌ Patch coverage is 95.49550% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.93%. Comparing base (4bb97ff) to head (776e687).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
...aid/src/rendering-util/rendering-elements/edges.js	81.48%	5 Missing ⚠️
...src/rendering-util/layout-algorithms/elk/render.ts	97.56%	4 Missing ⚠️
...c/rendering-util/layout-algorithms/common/index.ts	96.29%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8334      +/-   ##
===========================================
+ Coverage    80.87%   80.93%   +0.05%     
===========================================
  Files          620      620              
  Lines        85370    85534     +164     
  Branches     16206    18802    +2596     
===========================================
+ Hits         69047    69230     +183     
+ Misses       15305    15287      -18     
+ Partials      1018     1017       -1     

Flag	Coverage Δ
e2e	73.73% <84.34%> (+0.06%)	⬆️
unit	79.06% <95.49%> (+0.07%)	⬆️

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Files with missing lines	Coverage Δ
packages/mermaid/src/utils.ts	86.51% <100.00%> (+0.70%)	⬆️
...c/rendering-util/layout-algorithms/common/index.ts	95.61% <96.29%> (+7.08%)	⬆️
...src/rendering-util/layout-algorithms/elk/render.ts	96.17% <97.56%> (+0.14%)	⬆️
...aid/src/rendering-util/rendering-elements/edges.js	72.03% <81.48%> (+0.48%)	⬆️

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• .changeset/elk-class-cardinality-labels.md
• docs/config/setup/mermaid/interfaces/LayoutData.md
• e2e/rendering/class/classDiagram-elk-v3.spec.js
• packages/mermaid/src/rendering-util/layout-algorithms/common/index.ts
• packages/mermaid/src/rendering-util/layout-algorithms/elk/__tests__/terminalLabels.spec.ts
• packages/mermaid/src/rendering-util/layout-algorithms/elk/render.ts
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Build	Status	Details	Updated (UTC)
default (Inspect)	👍 Approved by Ashish Jain	19 changed, 1 ignored	Sep 28, 2026, 11:34 AM

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• packages/mermaid/src/rendering-util/layout-algorithms/elk/render.ts
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