fix(webgl): normalize nearly identical vertices before tessellation

src/webgl/p5.RendererGL.Immediate.js

@@ -436,6 +436,22 @@ p5.RendererGL.prototype._tesselateShape = function() {
       this.immediateMode.geometry.vertexNormals[i].z
     );
   }
+
+  // Snap nearly identical consecutive vertex coordinates to avoid libtess
+  // tessellation artifacts. Common with font.textToContours() output.
+  const stride = p5.RendererGL.prototype.tessyVertexSize;
+  const epsilon = 1e-6;
+  for (const contour of contours) {
+    for (let i = stride; i < contour.length; i += stride) {
+      if (Math.abs(contour[i] - contour[i - stride]) < epsilon) {
+        contour[i] = contour[i - stride];
+      }
+      if (Math.abs(contour[i + 1] - contour[i - stride + 1]) < epsilon) {
+        contour[i + 1] = contour[i - stride + 1];
+      }
+    }
+  }
+
   const polyTriangles = this._triangulate(contours);
   const originalVertices = this.immediateMode.geometry.vertices;
   this.immediateMode.geometry.vertices = [];

test/unit/visual/cases/webgl.js

@@ -141,4 +141,33 @@ visualSuite('WebGL', function() {
       screenshot();
     });
   });
+
+  visualSuite('Tessellation', function() {
+    visualTest(
+      'Handles nearly identical consecutive vertices from textToContours',
+      function(p5, screenshot) {
+        p5.createCanvas(50, 50, p5.WEBGL);
+        p5.background(220);
+        p5.fill(40);
+        p5.noStroke();
+
+        p5.beginShape();
+        p5.vertex(-20, -20);
+        p5.vertex(20, -20);
+        p5.vertex(20, 20);
+        p5.vertex(-20, 20);
+
+        // Inner contour (hole) — vertex 2 has x nudged by 1e-8
+        p5.beginContour();
+        p5.vertex(8, -8);
+        p5.vertex(8, 8);
+        p5.vertex(-8.00000001, 8);  // nearly identical x to next vertex
+        p5.vertex(-8, -8);
+        p5.endContour();
+        p5.endShape(p5.CLOSE);
+
+        screenshot();
+      }
+    );
+  });
 });

test/unit/visual/screenshots/WebGL/Tessellation/Handles nearly identical consecutive vertices from textToContours/metadata.json

@@ -0,0 +1,3 @@
+{
+  "numScreenshots": 1
+}