emscripten.patch

--- upstream/emscripten/src/library_webgl.js	2021-03-05 12:32:24.000000000 -0800
+++ upstream/emscripten/src/library_webgl.js	2022-05-02 10:19:02.461667100 -0700
@@ -3486,6 +3486,7 @@
   glDrawElements: function(mode, count, type, indices) {
 #if FULL_ES2
     var buf;
+    var vertices = 0;
     if (!GLctx.currentElementArrayBufferBinding) {
       var size = GL.calcBufLength(1, type, 0, count);
       buf = GL.getTempIndexBuffer(size);
@@ -3493,12 +3494,26 @@
       GLctx.bufferSubData(0x8893 /*GL_ELEMENT_ARRAY_BUFFER*/,
                                0,
                                HEAPU8.subarray(indices, indices + size));
+      
+      // Detecting vertex count if attributes' data should be loaded
+      var array_classes = {
+        '5121' /* GL_UNSIGNED_BYTE */: Uint8Array,
+        '5123' /* GL_UNSIGNED_SHORT */: Uint16Array,
+        '5125' /* GL_UNSIGNED_INT */: Uint32Array};
+      for (var i = 0; i < GL.currentContext.maxVertexAttribs; i++) {
+        var cb = GL.currentContext.clientBuffers[i];
+        if (cb.clientside && cb.enabled && array_classes[type]) {
+          vertices = Math.max.apply(null, new array_classes[type](HEAPU8.buffer, indices, count)) + 1;
+          break;
+        }
+      }
+      
       // the index is now 0
       indices = 0;
     }
 
     // bind any client-side buffers
-    GL.preDrawHandleClientVertexAttribBindings(count);
+    GL.preDrawHandleClientVertexAttribBindings(vertices);
 #endif
 
     GLctx.drawElements(mode, count, type, indices);