fix: keep mainUv effects out of the same auto-merged pass as convolution effects

src/EffectComposer.tsx

@@ -70,6 +70,9 @@ type ComposerState = {
 const isConvolution = (effect: Effect): boolean =>
   (effect.getAttributes() & EffectAttribute.CONVOLUTION) === EffectAttribute.CONVOLUTION
 
+// mainUv isn't a bitflag - detect it the same way postprocessing itself does.
+const hasMainUv = (effect: Effect): boolean => /mainUv/.test(effect.getFragmentShader() ?? '')
+
 // autoClear/toneMapping get force-set and never restored. Ref-counted per
 // (renderer, property) since composers can share a renderer.
 function createRendererPropertyGuard<K extends 'autoClear' | 'toneMapping'>(property: K) {
@@ -139,13 +142,10 @@ function disposeGeneratedPass(pass: Pass): void {
   disposePassWithoutEffects(pass)
 }
 
-// 'auto' (default): consecutive Effects share one EffectPass, same as vanilla
-// postprocessing allows by hand - a run may contain at most one convolution
-// Effect (e.g. DepthOfField), since the library throws if two land in the
-// same pass. 'all' merges through that limit too, same no-guardrail
-// treatment EffectGroup already gives its own children - multiple
-// convolution Effects in one run will throw at render time. 'none' gives
-// every Effect its own EffectPass.
+// 'auto' (default): consecutive Effects share one EffectPass, keeping
+// convolution and mainUv Effects apart since postprocessing throws if they
+// mix. 'all' merges through those limits too, no guardrails, same as
+// EffectGroup. 'none' gives every Effect its own EffectPass.
 function buildPasses(nodes: Array<Effect | Pass>, camera: Camera, mergeMode: 'auto' | 'all' | 'none'): Pass[] {
   const passes: Pass[] = []
 
@@ -155,14 +155,24 @@ function buildPasses(nodes: Array<Effect | Pass>, camera: Camera, mergeMode: 'au
     if (node instanceof Effect) {
       const effects: Effect[] = [node]
       let hasConvolution = isConvolution(node)
+      let hasMainUvEffect = hasMainUv(node)
 
       if (mergeMode !== 'none') {
         let next: Effect | Pass | undefined
         while ((next = nodes[i + 1]) instanceof Effect) {
           const nextIsConvolution = isConvolution(next)
-          if (mergeMode === 'auto' && hasConvolution && nextIsConvolution) break
+          const nextHasMainUv = hasMainUv(next)
+          if (
+            mergeMode === 'auto' &&
+            ((hasConvolution && nextIsConvolution) ||
+              (hasConvolution && nextHasMainUv) ||
+              (hasMainUvEffect && nextIsConvolution))
+          ) {
+            break
+          }
           effects.push(next)
           hasConvolution ||= nextIsConvolution
+          hasMainUvEffect ||= nextHasMainUv
           i++
         }
       }

src/tests/EffectComposer.test.tsx

@@ -47,11 +47,25 @@ class ConvolutionEffectTwo extends Effect {
   }
 }
 
+const MAIN_UV_SHADER = `
+void mainUv(inout vec2 uv) {
+  uv += 0.0;
+}
+${EFFECT_SHADER}
+`
+
+class MainUvEffect extends Effect {
+  constructor() {
+    super('MainUvEffect', MAIN_UV_SHADER)
+  }
+}
+
 const WrappedEffectA = wrapEffect(EffectA)
 const WrappedEffectB = wrapEffect(EffectB)
 const WrappedEffectC = wrapEffect(EffectC)
 const WrappedConvolutionEffect = wrapEffect(ConvolutionEffect)
 const WrappedConvolutionEffectTwo = wrapEffect(ConvolutionEffectTwo)
+const WrappedMainUvEffect = wrapEffect(MainUvEffect)
 
 afterEach(async () => {
   await React.act(async () => {
@@ -503,6 +517,53 @@ describe('EffectComposer', () => {
       expect(secondEffects.filter((e) => e instanceof ConvolutionEffect || e instanceof ConvolutionEffectTwo)).toHaveLength(1)
     })
 
+    it("'auto' splits a mainUv effect and a convolution effect into separate passes (mainUv first)", async () => {
+      const ref = React.createRef<EffectComposerImpl>()
+
+      await React.act(async () =>
+        root.render(
+          <EffectComposer ref={ref}>
+            <WrappedMainUvEffect />
+            <WrappedConvolutionEffect />
+          </EffectComposer>
+        )
+      )
+
+      const composer = await waitForComposer(ref)
+      expect(composer.passes.filter((pass) => pass instanceof EffectPass)).toHaveLength(2)
+    })
+
+    it("'auto' splits a mainUv effect and a convolution effect into separate passes (convolution first)", async () => {
+      const ref = React.createRef<EffectComposerImpl>()
+
+      await React.act(async () =>
+        root.render(
+          <EffectComposer ref={ref}>
+            <WrappedConvolutionEffect />
+            <WrappedMainUvEffect />
+          </EffectComposer>
+        )
+      )
+
+      const composer = await waitForComposer(ref)
+      expect(composer.passes.filter((pass) => pass instanceof EffectPass)).toHaveLength(2)
+    })
+
+    it("'all' throws at render time if that removes 'auto'-only protection against merging a mainUv effect with a convolution effect", async () => {
+      const ref = React.createRef<EffectComposerImpl>()
+
+      await expect(
+        React.act(async () =>
+          root.render(
+            <EffectComposer ref={ref} mergeMode="all">
+              <WrappedMainUvEffect />
+              <WrappedConvolutionEffect />
+            </EffectComposer>
+          )
+        )
+      ).rejects.toThrow('Effects that transform UVs are incompatible with convolution effects')
+    })
+
     it("'all' merges even a single convolution effect with neighbors, same as 'auto'", async () => {
       const ref = React.createRef<EffectComposerImpl>()