ADR: PrimeVue Fork Decision (#5230)

* ADR: Add PrimeVue fork decision record

Adds ADR-0003 documenting the decision to fork PrimeVue as a monorepo workspace package. Key rationale includes transform coordinate system conflicts and virtual canvas scroll interference that require component-level modifications.

* ADR: Reject PrimeVue fork decision

- Change status from Proposed to Rejected
- Document rationale: implementation complexity with dual monorepos,
  maintenance burden, alternative solutions available
- Add specific code citations and repository links
- Include alternative approach using shadcn/ui for selective replacement

docs/adr/0004-fork-primevue-ui-library.md

@@ -0,0 +1,62 @@
+# 4. Fork PrimeVue UI Library
+
+Date: 2025-08-27
+
+## Status
+
+Rejected
+
+## Context
+
+ComfyUI's frontend requires modifications to PrimeVue components that cannot be achieved through the library's customization APIs. Two specific technical incompatibilities have been identified with the transform-based canvas architecture:
+
+**Screen Coordinate Hit-Testing Conflicts:**
+- PrimeVue components use `getBoundingClientRect()` for screen coordinate calculations that don't account for CSS transforms
+- The Slider component directly uses raw `pageX/pageY` coordinates ([lines 102-103](https://github.com/primefaces/primevue/blob/master/packages/primevue/src/slider/Slider.vue#L102-L103)) without transform-aware positioning
+- This breaks interaction in transformed coordinate spaces where screen coordinates don't match logical element positions
+
+**Virtual Canvas Scroll Interference:**
+- LiteGraph's infinite canvas uses scroll coordinates semantically for graph navigation via the `DragAndScale` coordinate system
+- PrimeVue overlay components automatically trigger `scrollIntoView` behavior which interferes with this virtual positioning
+- This issue is documented in [PrimeVue discussion #4270](https://github.com/orgs/primefaces/discussions/4270) where the feature request was made to disable this behavior
+
+**Historical Overlay Issues:**
+- Previous z-index positioning conflicts required manual workarounds (commit `6d4eafb0`) where PrimeVue Dialog components needed `autoZIndex: false` and custom mask styling, later resolved by removing PrimeVue's automatic z-index management entirely
+
+**Minimal Update Overhead:**
+- Analysis of git history shows only 2 PrimeVue version updates in 2+ years, indicating that upstream sync overhead is negligible for this project
+
+**Future Interaction System Requirements:**
+- The ongoing canvas architecture evolution will require more granular control over component interaction and event handling as the transform-based system matures
+- Predictable need for additional component modifications beyond current identified issues
+
+## Decision
+
+We will **NOT** fork PrimeVue. After evaluation, forking was determined to be unnecessarily complex and costly.
+
+**Rationale for Rejection:**
+
+- **Significant Implementation Complexity**: PrimeVue is structured as a monorepo ([primefaces/primevue](https://github.com/primefaces/primevue)) with significant code in a separate monorepo ([PrimeUIX](https://github.com/primefaces/primeuix)). Forking would require importing both repositories whole and selectively pruning or exempting components from our workspace tooling, adding substantial complexity.
+
+- **Alternative Solutions Available**: The modifications we identified (e.g., scroll interference issues, coordinate system conflicts) have less costly solutions that don't require maintaining a full fork. For example, coordinate issues could be addressed through event interception and synthetic event creation with scaled values.
+
+- **Maintenance Burden**: Ongoing maintenance and upgrades would be very painful, requiring manual conflict resolution and keeping pace with upstream changes across multiple repositories.
+
+- **Limited Tooling Support**: There isn't adequate tooling that provides the granularity needed to cleanly manage a PrimeVue fork within our existing infrastructure.
+
+## Consequences
+
+### Alternative Approach
+
+- **Use PrimeVue as External Dependency**: Continue using PrimeVue as a standard npm dependency
+- **Targeted Workarounds**: Implement specific solutions for identified issues (coordinate system conflicts, scroll interference) without forking the entire library
+- **Selective Component Replacement**: Use libraries like shadcn/ui to replace specific problematic PrimeVue components and adjust them to match our design system
+- **Upstream Engagement**: Continue engaging with PrimeVue community for feature requests and bug reports
+- **Maintain Flexibility**: Preserve ability to upgrade PrimeVue versions without fork maintenance overhead
+
+## Notes
+
+- Technical issues documented in the Context section remain valid concerns
+- Solutions will be pursued through targeted fixes rather than wholesale forking
+- Future re-evaluation possible if PrimeVue's architecture significantly changes or if alternative tooling becomes available
+- This decision prioritizes maintainability and development velocity over maximum customization control

docs/adr/README.md

@@ -13,6 +13,7 @@ An Architecture Decision Record captures an important architectural decision mad
 | [0001](0001-merge-litegraph-into-frontend.md) | Merge LiteGraph.js into ComfyUI Frontend | Accepted | 2025-08-05 |
 | [0002](0002-monorepo-conversion.md) | Restructure as a Monorepo | Accepted | 2025-08-25 |
 | [0003](0003-crdt-based-layout-system.md) | Centralized Layout Management with CRDT | Proposed | 2025-08-27 |
+| [0004](0004-fork-primevue-ui-library.md) | Fork PrimeVue UI Library | Rejected | 2025-08-27 |
 
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