A wrongly wound triangle is not drawn inside out, it is not drawn at all

Fact

With back-face culling enabled, whether a triangle is drawn is decided by the sign of its projected area — that is, by the order of its three vertices. A triangle wound the wrong way is discarded before rasterisation. There is no error, no warning, no z-fighting and no flipped shading. The geometry is not there.

This is not the intuition the name suggests. "Back face" implies something that would have been drawn facing away; what actually happens to a single-sided flat object is that it disappears from one side and is perfectly normal from the other.

Why it matters

The symptom is a missing feature rather than a broken-looking one, so the investigation starts in the wrong half of the program. A highlight that never appears reads as "the state was never set", which sends you through the event handlers, the uniforms and the code that decides when to show it — all of which are correct.

It also fails selectively in a way that reinforces the wrong theory. Meshes built by one helper appear while meshes from a sibling helper do not, because their index orders happen to differ by one swap, so the bug looks like it belongs to whatever those particular meshes represent.

How to apply

• Assert winding where meshes are built, not where they are drawn. For every triangle, cross(v1 - v0, v2 - v0) must point along the face normal you intend. It is a pure function of the mesh, so it is cheap to test and it catches the fault at its source.
• Turn culling off for single-sided flat art — overlays, decals, ground quads, UI planes. There is no interior to hide, so culling buys nothing and risks losing the whole object.
• When something is missing rather than wrong, rule out culling first: disabling it for one frame distinguishes a geometry fault from a state fault in seconds.
• Remember that winding is relative: frontFace can be set to either CW or CCW, and a mesh that is correct in one renderer can vanish in another.