From 051df1e1cac7a4492e1fb6493634e3eee22fb2a6 Mon Sep 17 00:00:00 2001
From: Nixxx19 <nixxspeaks@gmail.com>
Date: Sun, 22 Feb 2026 17:45:38 +0530
Subject: [PATCH] fix: prevent browser freeze when tessellating >50k vertices

---
 src/webgl/p5.RendererGL.Immediate.js | 17 +++++++
 test/unit/webgl/p5.RendererGL.js     | 67 ++++++++++++++++++++++++++++
 2 files changed, 84 insertions(+)

diff --git a/src/webgl/p5.RendererGL.Immediate.js b/src/webgl/p5.RendererGL.Immediate.js
index 31ce48f630..915db04537 100644
--- a/src/webgl/p5.RendererGL.Immediate.js
+++ b/src/webgl/p5.RendererGL.Immediate.js
@@ -313,6 +313,23 @@ p5.RendererGL.prototype._processVertices = function(mode) {
     this.immediateMode.shapeMode !== constants.LINES;
 
   if (shouldTess) {
+    // libtess can't handle >65k vertices and will freeze the browser
+    const vertexCount = this.immediateMode.geometry.vertices.length;
+    const MAX_SAFE_TESSELATION_VERTICES = 50000;
+
+    if (vertexCount > MAX_SAFE_TESSELATION_VERTICES) {
+      p5._friendlyError(
+        'p5.js WebGL: Tessellation warning',
+        `Attempting to tessellate a shape with ${vertexCount} vertices. ` +
+        `Tessellation of shapes with more than ${MAX_SAFE_TESSELATION_VERTICES} vertices ` +
+        'may cause the browser to freeze. Consider reducing the number of vertices ' +
+        'or using a different shape mode (e.g., TRIANGLES, TRIANGLE_STRIP) instead of TESS.'
+      );
+      // skip tessellation to prevent freeze, use TRIANGLE_FAN as fallback
+      this.immediateMode.shapeMode = constants.TRIANGLE_FAN;
+      return;
+    }
+
     this._tesselateShape();
   }
 };
diff --git a/test/unit/webgl/p5.RendererGL.js b/test/unit/webgl/p5.RendererGL.js
index 3c7e3df1a2..26e0d24b84 100644
--- a/test/unit/webgl/p5.RendererGL.js
+++ b/test/unit/webgl/p5.RendererGL.js
@@ -1686,6 +1686,73 @@ suite('p5.RendererGL', function() {
       done();
     });
 
+    test('TESS mode warns and prevents freeze for >50k vertices', function(done) {
+      var renderer = myp5.createCanvas(10, 10, myp5.WEBGL);
+      var friendlyErrorSpy = sinon.spy(p5, '_friendlyError');
+
+      renderer.beginShape(myp5.TESS);
+      for (let i = 0; i < 60000; i++) {
+        renderer.vertex(
+          Math.random() * 10 - 5,
+          Math.random() * 10 - 5,
+          Math.random() * 10 - 5
+        );
+      }
+      renderer.endShape();
+
+      assert.isTrue(
+        friendlyErrorSpy.called,
+        'p5._friendlyError should be called for large vertex count'
+      );
+
+      const warningCall = friendlyErrorSpy.getCalls().find(call =>
+        call.args[0] && call.args[0].includes('Tessellation warning')
+      );
+      assert.isDefined(
+        warningCall,
+        'Tessellation warning should be shown'
+      );
+
+      assert.equal(
+        renderer.immediateMode.shapeMode,
+        myp5.TRIANGLE_FAN,
+        'Shape mode should be changed to TRIANGLE_FAN when tessellation is skipped'
+      );
+
+      friendlyErrorSpy.restore();
+      done();
+    });
+
+    test('TESS mode works normally for <50k vertices', function(done) {
+      var renderer = myp5.createCanvas(10, 10, myp5.WEBGL);
+      var friendlyErrorSpy = sinon.spy(p5, '_friendlyError');
+
+      // use a simple shape that tessellates quickly
+      renderer.beginShape(myp5.TESS);
+      renderer.vertex(-10, -10, 0);
+      renderer.vertex(10, -10, 0);
+      renderer.vertex(10, 10, 0);
+      renderer.vertex(-10, 10, 0);
+      renderer.endShape(myp5.CLOSE);
+
+      const warningCall = friendlyErrorSpy.getCalls().find(call =>
+        call.args[0] && call.args[0].includes('Tessellation warning')
+      );
+      assert.isUndefined(
+        warningCall,
+        'No tessellation warning should be shown for <50k vertices'
+      );
+
+      assert.equal(
+        renderer.immediateMode.shapeMode,
+        myp5.TRIANGLES,
+        'Shape mode should be TRIANGLES after normal tessellation'
+      );
+
+      friendlyErrorSpy.restore();
+      done();
+    });
+
     test('TESS does not affect stroke colors', function(done) {
       var renderer = myp5.createCanvas(10, 10, myp5.WEBGL);