dm-viewport-fallback-rule

name	DM Viewport Fallback Rule
description	Enforces the "No Silent Fallbacks" rule for workplanes when modeling.

DM Viewport Fallback Rule

Core Rule: No Silent Fallbacks

This project strictly enforces that tools behavior should never depend on hidden or implicitly cached state that the user cannot see.

When a user triggers a tool click, the tool casts a ray through the viewport. If the ray hits a face on existing geometry, it aligns the workplane to that face. If it hits an explicit visual DMWorkPlane, it snaps to it.

If the ray hits empty space, the fall-back behavior must be completely stateless:

• CORRECT: Assume the camera-facing viewport plane passing through the origin (or camera focus).
• WRONG: Re-use the "last working plane" that visual tools used on previous clicks. This is a "silent fallback" that leaves the user confused when they start drawing and their object snaps to an invisible angled grid from a previous tool session.

Subclassing Guidelines

When creating a new modeling tool (e.g. subclassing PrimitiveCreatorBase or NURBSPrimitiveCreator), you must:

1. NEVER declare or use a class-level variable like _last_working_plane.
2. Ensure that empty-space clicks correctly fall back to the viewport aligned plane natively provided by ViewProjector.get_base_plane().
3. Always trust get_mouse_plane_pt()'s fallback; do not attempt to cache it across tool invocations unless an explicit, user-facing persistent object (like a DMWorkPlane) dictates it.