diff --git a/src/lib/libhtml5.js b/src/lib/libhtml5.js
index 0d110367b4fe9..6c28e9b5471f7 100644
--- a/src/lib/libhtml5.js
+++ b/src/lib/libhtml5.js
@@ -258,18 +258,15 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.keyEvent ||= _malloc({{{ C_STRUCTS.EmscriptenKeyboardEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenKeyboardEvent.__size__ }}};
+    JSEvents.keyEvent ||= _malloc(eventSize);
 
     var keyEventHandlerFunc = (e) => {
 #if ASSERTIONS
       assert(e);
 #endif
 
-#if PTHREADS
-      var keyEventData = targetThread ? _malloc({{{ C_STRUCTS.EmscriptenKeyboardEvent.__size__ }}}) : JSEvents.keyEvent; // This allocated block is passed as satellite data to the proxied function call, so the call frees up the data block when done.
-#else
       var keyEventData = JSEvents.keyEvent;
-#endif
       {{{ makeSetValue('keyEventData', C_STRUCTS.EmscriptenKeyboardEvent.timestamp, 'e.timeStamp', 'double') }}};
 
       var idx = {{{ getHeapOffset('keyEventData', 'i32') }}};
@@ -289,7 +286,7 @@ var LibraryHTML5 = {
       stringToUTF8(e.locale || '', keyEventData + {{{ C_STRUCTS.EmscriptenKeyboardEvent.locale }}}, {{{ cDefs.EM_HTML5_SHORT_STRING_LEN_BYTES }}});
 
 #if PTHREADS
-      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, keyEventData, userData);
+      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, keyEventData, eventSize, userData);
       else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, keyEventData, userData)) e.preventDefault();
@@ -480,7 +477,8 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.mouseEvent ||= _malloc({{{ C_STRUCTS.EmscriptenMouseEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenMouseEvent.__size__ }}};
+    JSEvents.mouseEvent ||= _malloc(eventSize);
     target = findEventTarget(target);
 
     var mouseEventHandlerFunc = (e = event) => {
@@ -489,9 +487,7 @@ var LibraryHTML5 = {
 
 #if PTHREADS
       if (targetThread) {
-        var mouseEventData = _malloc({{{ C_STRUCTS.EmscriptenMouseEvent.__size__ }}}); // This allocated block is passed as satellite data to the proxied function call, so the call frees up the data block when done.
-        fillMouseEventData(mouseEventData, e, target);
-        __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, mouseEventData, userData);
+        __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, JSEvents.mouseEvent, eventSize, userData);
       } else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, JSEvents.mouseEvent, userData)) e.preventDefault();
@@ -572,22 +568,19 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.wheelEvent ||= _malloc({{{ C_STRUCTS.EmscriptenWheelEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenWheelEvent.__size__ }}};
+    JSEvents.wheelEvent ||= _malloc(eventSize)
 
     // The DOM Level 3 events spec event 'wheel'
     var wheelHandlerFunc = (e = event) => {
-#if PTHREADS
-      var wheelEvent = targetThread ? _malloc({{{ C_STRUCTS.EmscriptenWheelEvent.__size__ }}}) : JSEvents.wheelEvent; // This allocated block is passed as satellite data to the proxied function call, so the call frees up the data block when done.
-#else
       var wheelEvent = JSEvents.wheelEvent;
-#endif
       fillMouseEventData(wheelEvent, e, target);
       {{{ makeSetValue('wheelEvent', C_STRUCTS.EmscriptenWheelEvent.deltaX, 'e["deltaX"]', 'double') }}};
       {{{ makeSetValue('wheelEvent', C_STRUCTS.EmscriptenWheelEvent.deltaY, 'e["deltaY"]', 'double') }}};
       {{{ makeSetValue('wheelEvent', C_STRUCTS.EmscriptenWheelEvent.deltaZ, 'e["deltaZ"]', 'double') }}};
       {{{ makeSetValue('wheelEvent', C_STRUCTS.EmscriptenWheelEvent.deltaMode, 'e["deltaMode"]', 'i32') }}};
 #if PTHREADS
-      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, wheelEvent, userData);
+      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, wheelEvent, eventSize, userData);
       else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, wheelEvent, userData)) e.preventDefault();
@@ -624,7 +617,8 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.uiEvent ||= _malloc({{{ C_STRUCTS.EmscriptenUiEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenUiEvent.__size__ }}};
+    JSEvents.uiEvent ||= _malloc(eventSize);
 
 #if DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR
     target = findEventTarget(target);
@@ -650,11 +644,7 @@ var LibraryHTML5 = {
         // During a page unload 'body' can be null, with "Cannot read property 'clientWidth' of null" being thrown
         return;
       }
-#if PTHREADS
-      var uiEvent = targetThread ? _malloc({{{ C_STRUCTS.EmscriptenUiEvent.__size__ }}}) : JSEvents.uiEvent;
-#else
       var uiEvent = JSEvents.uiEvent;
-#endif
       {{{ makeSetValue('uiEvent', C_STRUCTS.EmscriptenUiEvent.detail, '0', 'i32') }}}; // always zero for resize and scroll
       {{{ makeSetValue('uiEvent', C_STRUCTS.EmscriptenUiEvent.documentBodyClientWidth, 'b.clientWidth', 'i32') }}};
       {{{ makeSetValue('uiEvent', C_STRUCTS.EmscriptenUiEvent.documentBodyClientHeight, 'b.clientHeight', 'i32') }}};
@@ -665,7 +655,7 @@ var LibraryHTML5 = {
       {{{ makeSetValue('uiEvent', C_STRUCTS.EmscriptenUiEvent.scrollTop, 'pageXOffset | 0', 'i32') }}}; // scroll offsets are float
       {{{ makeSetValue('uiEvent', C_STRUCTS.EmscriptenUiEvent.scrollLeft, 'pageYOffset | 0', 'i32') }}};
 #if PTHREADS
-      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, uiEvent, userData);
+      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, uiEvent, eventSize, userData);
       else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, uiEvent, userData)) e.preventDefault();
@@ -697,22 +687,19 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.focusEvent ||= _malloc({{{ C_STRUCTS.EmscriptenFocusEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenFocusEvent.__size__ }}};
+    JSEvents.focusEvent ||= _malloc(eventSize);
 
     var focusEventHandlerFunc = (e = event) => {
       var nodeName = JSEvents.getNodeNameForTarget(e.target);
       var id = e.target.id ? e.target.id : '';
 
-#if PTHREADS
-      var focusEvent = targetThread ? _malloc({{{ C_STRUCTS.EmscriptenFocusEvent.__size__ }}}) : JSEvents.focusEvent;
-#else
       var focusEvent = JSEvents.focusEvent;
-#endif
       stringToUTF8(nodeName, focusEvent + {{{ C_STRUCTS.EmscriptenFocusEvent.nodeName }}}, {{{ cDefs.EM_HTML5_LONG_STRING_LEN_BYTES }}});
       stringToUTF8(id, focusEvent + {{{ C_STRUCTS.EmscriptenFocusEvent.id }}}, {{{ cDefs.EM_HTML5_LONG_STRING_LEN_BYTES }}});
 
 #if PTHREADS
-      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, focusEvent, userData);
+      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, focusEvent, eventSize, userData);
       else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, focusEvent, userData)) e.preventDefault();
@@ -761,16 +748,15 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.deviceOrientationEvent ||= _malloc({{{ C_STRUCTS.EmscriptenDeviceOrientationEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenDeviceOrientationEvent.__size__ }}};
+    JSEvents.deviceOrientationEvent ||= _malloc(eventSize);
 
     var deviceOrientationEventHandlerFunc = (e = event) => {
       fillDeviceOrientationEventData(JSEvents.deviceOrientationEvent, e, target); // TODO: Thread-safety with respect to emscripten_get_deviceorientation_status()
 
 #if PTHREADS
       if (targetThread) {
-        var deviceOrientationEvent = _malloc({{{ C_STRUCTS.EmscriptenDeviceOrientationEvent.__size__ }}});
-        fillDeviceOrientationEventData(deviceOrientationEvent, e, target);
-        __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, deviceOrientationEvent, userData);
+        __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, JSEvents.deviceOrientationEvent, eventSize, userData);
       } else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, JSEvents.deviceOrientationEvent, userData)) e.preventDefault();
@@ -832,16 +818,15 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.deviceMotionEvent ||= _malloc({{{ C_STRUCTS.EmscriptenDeviceMotionEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenDeviceMotionEvent.__size__ }}};
+    JSEvents.deviceMotionEvent ||= _malloc(eventSize);
 
     var deviceMotionEventHandlerFunc = (e = event) => {
       fillDeviceMotionEventData(JSEvents.deviceMotionEvent, e, target); // TODO: Thread-safety with respect to emscripten_get_devicemotion_status()
 
 #if PTHREADS
       if (targetThread) {
-        var deviceMotionEvent = _malloc({{{ C_STRUCTS.EmscriptenDeviceMotionEvent.__size__ }}});
-        fillDeviceMotionEventData(deviceMotionEvent, e, target);
-        __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, deviceMotionEvent, userData);
+        __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, JSEvents.deviceMotionEvent, eventSize, userData);
       } else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, JSEvents.deviceMotionEvent, userData)) e.preventDefault();
@@ -915,19 +900,15 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.orientationChangeEvent ||= _malloc({{{ C_STRUCTS.EmscriptenOrientationChangeEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenOrientationChangeEvent.__size__ }}};
+    JSEvents.orientationChangeEvent ||= _malloc(eventSize);
 
     var orientationChangeEventHandlerFunc = (e = event) => {
-#if PTHREADS
-      var orientationChangeEvent = targetThread ? _malloc({{{ C_STRUCTS.EmscriptenOrientationChangeEvent.__size__ }}}) : JSEvents.orientationChangeEvent;
-#else
       var orientationChangeEvent = JSEvents.orientationChangeEvent;
-#endif
-
       fillOrientationChangeEventData(orientationChangeEvent);
 
 #if PTHREADS
-      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, orientationChangeEvent, userData);
+      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, orientationChangeEvent, eventSize, userData);
       else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, orientationChangeEvent, userData)) e.preventDefault();
@@ -1026,19 +1007,15 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.fullscreenChangeEvent ||= _malloc({{{ C_STRUCTS.EmscriptenFullscreenChangeEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenFullscreenChangeEvent.__size__ }}};
+    JSEvents.fullscreenChangeEvent ||= _malloc(eventSize);
 
     var fullscreenChangeEventhandlerFunc = (e = event) => {
-#if PTHREADS
-      var fullscreenChangeEvent = targetThread ? _malloc({{{ C_STRUCTS.EmscriptenFullscreenChangeEvent.__size__ }}}) : JSEvents.fullscreenChangeEvent;
-#else
       var fullscreenChangeEvent = JSEvents.fullscreenChangeEvent;
-#endif
-
       fillFullscreenChangeEventData(fullscreenChangeEvent);
 
 #if PTHREADS
-      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, fullscreenChangeEvent, userData);
+      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, fullscreenChangeEvent, eventSize, userData);
       else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, fullscreenChangeEvent, userData)) e.preventDefault();
@@ -1528,18 +1505,15 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.pointerlockChangeEvent ||= _malloc({{{ C_STRUCTS.EmscriptenPointerlockChangeEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenPointerlockChangeEvent.__size__ }}};
+    JSEvents.pointerlockChangeEvent ||= _malloc(eventSize);
 
     var pointerlockChangeEventHandlerFunc = (e = event) => {
-#if PTHREADS
-      var pointerlockChangeEvent = targetThread ? _malloc({{{ C_STRUCTS.EmscriptenPointerlockChangeEvent.__size__ }}}) : JSEvents.pointerlockChangeEvent;
-#else
       var pointerlockChangeEvent = JSEvents.pointerlockChangeEvent;
-#endif
       fillPointerlockChangeEventData(pointerlockChangeEvent);
 
 #if PTHREADS
-      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, pointerlockChangeEvent, userData);
+      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, pointerlockChangeEvent, eventSize, userData);
       else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, pointerlockChangeEvent, userData)) e.preventDefault();
@@ -1725,19 +1699,15 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.visibilityChangeEvent ||= _malloc({{{ C_STRUCTS.EmscriptenVisibilityChangeEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenVisibilityChangeEvent.__size__ }}};
+    JSEvents.visibilityChangeEvent ||= _malloc(eventSize);
 
     var visibilityChangeEventHandlerFunc = (e = event) => {
-#if PTHREADS
-      var visibilityChangeEvent = targetThread ? _malloc({{{ C_STRUCTS.EmscriptenVisibilityChangeEvent.__size__ }}}) : JSEvents.visibilityChangeEvent;
-#else
       var visibilityChangeEvent = JSEvents.visibilityChangeEvent;
-#endif
-
       fillVisibilityChangeEventData(visibilityChangeEvent);
 
 #if PTHREADS
-      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, visibilityChangeEvent, userData);
+      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, visibilityChangeEvent, eventSize, userData);
       else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, visibilityChangeEvent, userData)) e.preventDefault();
@@ -1780,7 +1750,8 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.touchEvent ||= _malloc({{{ C_STRUCTS.EmscriptenTouchEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenTouchEvent.__size__ }}};
+    JSEvents.touchEvent ||= _malloc(eventSize);
 
     target = findEventTarget(target);
 
@@ -1809,11 +1780,7 @@ var LibraryHTML5 = {
         touches[t.identifier].onTarget = 1;
       }
 
-#if PTHREADS
-      var touchEvent = targetThread ? _malloc({{{ C_STRUCTS.EmscriptenTouchEvent.__size__ }}}) : JSEvents.touchEvent;
-#else
       var touchEvent = JSEvents.touchEvent;
-#endif
       {{{ makeSetValue('touchEvent', C_STRUCTS.EmscriptenTouchEvent.timestamp, 'e.timeStamp', 'double') }}};
       HEAP8[touchEvent + {{{ C_STRUCTS.EmscriptenTouchEvent.ctrlKey }}}] = e.ctrlKey;
       HEAP8[touchEvent + {{{ C_STRUCTS.EmscriptenTouchEvent.shiftKey }}}] = e.shiftKey;
@@ -1852,7 +1819,7 @@ var LibraryHTML5 = {
       {{{ makeSetValue('touchEvent', C_STRUCTS.EmscriptenTouchEvent.numTouches, 'numTouches', 'i32') }}};
 
 #if PTHREADS
-      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, touchEvent, userData);
+      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, touchEvent, eventSize, userData);
       else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, touchEvent, userData)) e.preventDefault();
@@ -1927,18 +1894,15 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.gamepadEvent ||= _malloc({{{ C_STRUCTS.EmscriptenGamepadEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenGamepadEvent.__size__ }}};
+    JSEvents.gamepadEvent ||= _malloc(eventSize);
 
     var gamepadEventHandlerFunc = (e = event) => {
-#if PTHREADS
-      var gamepadEvent = targetThread ? _malloc({{{ C_STRUCTS.EmscriptenGamepadEvent.__size__ }}}) : JSEvents.gamepadEvent;
-#else
       var gamepadEvent = JSEvents.gamepadEvent;
-#endif
       fillGamepadEventData(gamepadEvent, e["gamepad"]);
 
 #if PTHREADS
-      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, gamepadEvent, userData);
+      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, gamepadEvent, eventSize, userData);
       else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, gamepadEvent, userData)) e.preventDefault();
@@ -2069,18 +2033,15 @@ var LibraryHTML5 = {
 #if PTHREADS
     targetThread = JSEvents.getTargetThreadForEventCallback(targetThread);
 #endif
-    JSEvents.batteryEvent ||= _malloc({{{ C_STRUCTS.EmscriptenBatteryEvent.__size__ }}});
+    var eventSize = {{{ C_STRUCTS.EmscriptenBatteryEvent.__size__ }}};
+    JSEvents.batteryEvent ||= _malloc(eventSize)
 
     var batteryEventHandlerFunc = (e = event) => {
-#if PTHREADS
-      var batteryEvent = targetThread ? _malloc({{{ C_STRUCTS.EmscriptenBatteryEvent.__size__ }}}) : JSEvents.batteryEvent;
-#else
       var batteryEvent = JSEvents.batteryEvent;
-#endif
       fillBatteryEventData(batteryEvent, battery);
 
 #if PTHREADS
-      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, batteryEvent, userData);
+      if (targetThread) __emscripten_run_callback_on_thread(targetThread, callbackfunc, eventTypeId, batteryEvent, eventSize, userData);
       else
 #endif
       if ({{{ makeDynCall('iipp', 'callbackfunc') }}}(eventTypeId, batteryEvent, userData)) e.preventDefault();
diff --git a/system/lib/html5/callback.c b/system/lib/html5/callback.c
index 23925e4ee5bc7..981a04f2013df 100644
--- a/system/lib/html5/callback.c
+++ b/system/lib/html5/callback.c
@@ -5,6 +5,7 @@
  * found in the LICENSE file.
  */
 #include <assert.h>
+#include <string.h>
 #include <emscripten/html5.h>
 
 #include "emscripten_internal.h"
@@ -14,8 +15,8 @@ typedef bool (*event_callback)(int event_type, void *event_data __attribute__((n
 typedef struct callback_args_t {
   event_callback callback;
   int event_type;
-  void *event_data;
   void *user_data;
+  uint8_t event_data[];
 } callback_args_t;
 
 static void do_callback(void* arg) {
@@ -28,13 +29,14 @@ void _emscripten_run_callback_on_thread(pthread_t t,
                                         event_callback f,
                                         int event_type,
                                         void* event_data,
+                                        size_t event_data_size,
                                         void* user_data) {
   em_proxying_queue* q = emscripten_proxy_get_system_queue();
-  callback_args_t* arg = malloc(sizeof(callback_args_t));
+  callback_args_t* arg = malloc(sizeof(callback_args_t) + event_data_size);
   arg->callback = f;
   arg->event_type = event_type;
-  arg->event_data = event_data;
   arg->user_data = user_data;
+  memcpy(arg->event_data, event_data, event_data_size);
 
   if (!emscripten_proxy_async(q, t, do_callback, arg)) {
     assert(false && "emscripten_proxy_async failed");
diff --git a/test/codesize/test_codesize_hello_dylink_all.json b/test/codesize/test_codesize_hello_dylink_all.json
index 37009f94946f8..16afb2eccaf70 100644
--- a/test/codesize/test_codesize_hello_dylink_all.json
+++ b/test/codesize/test_codesize_hello_dylink_all.json
@@ -1,7 +1,7 @@
 {
   "a.out.js": 245483,
-  "a.out.nodebug.wasm": 573907,
-  "total": 819390,
+  "a.out.nodebug.wasm": 573921,
+  "total": 819404,
   "sent": [
     "IMG_Init",
     "IMG_Load",
diff --git a/tools/emscripten.py b/tools/emscripten.py
index 87ca13f4d1546..a94b2401dde96 100644
--- a/tools/emscripten.py
+++ b/tools/emscripten.py
@@ -1145,7 +1145,7 @@ def create_pointer_conversion_wrappers(metadata):
     'emscripten_main_runtime_thread_id': 'p',
     '_emscripten_set_offscreencanvas_size_on_thread': '_pp__',
     'fileno': '_p',
-    '_emscripten_run_callback_on_thread': '_pp_pp',
+    '_emscripten_run_callback_on_thread': '_pp_ppp',
     '_emscripten_find_dylib': 'ppppp',
   }