The Issue

Currently, LibMultiPart uses a WorldRenderer mixin in order to draw custom block outlines for multipart blocks. The problem with this is that most custom Minecraft render backend mods, like Canvas, don't actually use WorldRenderer when rendering. This means that mixins into WorldRenderer never get called when custom render backends are enabled.

Screenshots

Block Outlines of WiredRedstone Wires in Canvas

Block Outlines of WiredRedstone Wires in the Vanilla Renderer

Potential Fix

The Fabric API provides an event for doing custom block outline rendering, allowing mods render their own block outlines and optionally cancel the default ones. This event is accessible via the WorldRenderEvents.BLOCK_OUTLINE event bus.

Pull Request

I have a fix for this issue written and I intend on submitting it as a pull request shortly.