diff --git a/src/compiler/compile.js b/src/compiler/compile.js
index fb43164504..b5edb81352 100644
--- a/src/compiler/compile.js
+++ b/src/compiler/compile.js
@@ -1,4 +1,4 @@
-const IRGenerator = require('./irgen');
+const {IRGenerator} = require('./irgen');
 const JSGenerator = require('./jsgen');
 
 const compile = thread => {
diff --git a/src/compiler/irgen.js b/src/compiler/irgen.js
index 40f007a75d..c3679fb41a 100644
--- a/src/compiler/irgen.js
+++ b/src/compiler/irgen.js
@@ -1693,4 +1693,7 @@ class IRGenerator {
     }
 }
 
-module.exports = IRGenerator;
+module.exports = {
+    ScriptTreeGenerator,
+    IRGenerator
+};
diff --git a/src/engine/thread.js b/src/engine/thread.js
index a4e55504f9..c8cf6cb1c1 100644
--- a/src/engine/thread.js
+++ b/src/engine/thread.js
@@ -514,4 +514,7 @@ class Thread {
     }
 }
 
+// for extensions
+Thread._StackFrame = _StackFrame;
+
 module.exports = Thread;
diff --git a/src/extensions/scratch3_pen/index.js b/src/extensions/scratch3_pen/index.js
index 16c09dfb61..b1b8eace4a 100644
--- a/src/extensions/scratch3_pen/index.js
+++ b/src/extensions/scratch3_pen/index.js
@@ -298,6 +298,16 @@ class Scratch3PenBlocks {
             }),
             blockIconURI: blockIconURI,
             blocks: [
+                // tw: additional message when on the stage for clarity
+                {
+                    blockType: BlockType.LABEL,
+                    text: formatMessage({
+                        id: 'tw.pen.stageSelected',
+                        default: 'Stage selected: less pen blocks',
+                        description: 'Label that appears in the Pen category when the stage is selected'
+                    }),
+                    filter: [TargetType.STAGE]
+                },
                 {
                     opcode: 'clear',
                     blockType: BlockType.COMMAND,
diff --git a/src/util/base64-util.js b/src/util/base64-util.js
index 1325291707..19165c980b 100644
--- a/src/util/base64-util.js
+++ b/src/util/base64-util.js
@@ -20,10 +20,13 @@ class Base64Util {
 
     /**
      * Convert a Uint8Array to a base64 encoded string.
-     * @param {Uint8Array} array - the array to convert.
+     * @param {Uint8Array|Array<number>} array - the array to convert.
      * @return {string} - the base64 encoded string.
      */
     static uint8ArrayToBase64 (array) {
+        if (Array.isArray(array)) {
+            array = new Uint8Array(array);
+        }
         let binary = '';
         const len = array.byteLength;
         for (let i = 0; i < len; i++) {
diff --git a/src/virtual-machine.js b/src/virtual-machine.js
index fd218be46b..e1a1fcdea9 100644
--- a/src/virtual-machine.js
+++ b/src/virtual-machine.js
@@ -220,7 +220,17 @@ class VirtualMachine extends EventEmitter {
         this.exports = {
             Sprite,
             RenderedTarget,
-            JSZip
+            JSZip,
+
+            i_will_not_ask_for_help_when_these_break: () => {
+                console.warn('You are using unsupported APIs. WHEN your code breaks, do not expect help.');
+                return ({
+                    JSGenerator: require('./compiler/jsgen.js'),
+                    IRGenerator: require('./compiler/irgen.js').IRGenerator,
+                    ScriptTreeGenerator: require('./compiler/irgen.js').ScriptTreeGenerator,
+                    Thread: require('./engine/thread.js')
+                });
+            }
         };
     }
 
diff --git a/test/fixtures/tw-block-returning-promise-like.sb3 b/test/fixtures/tw-block-returning-promise-like.sb3
index d7c232ca21..4ef6f8076e 100644
Binary files a/test/fixtures/tw-block-returning-promise-like.sb3 and b/test/fixtures/tw-block-returning-promise-like.sb3 differ
diff --git a/test/integration/tw_block_returning_promise_like.js b/test/integration/tw_block_returning_promise_like.js
index a0e95e8c3e..e212ef4266 100644
--- a/test/integration/tw_block_returning_promise_like.js
+++ b/test/integration/tw_block_returning_promise_like.js
@@ -21,13 +21,13 @@ class TestExtension {
         };
     }
     test () {
-        // returns a PromiseLike that calls handler immediately.
+        // returns a PromiseLike that calls handler immediately instead of in next microtask.
         const promise = {
             then (callbackFn) {
                 callbackFn();
                 return promise;
             }
-            // intentionally omit catch() as that is not part of PromiseLike
+            // intentionally omit catch() as that is not part of PromiseLike, so it should not be used
         };
         return promise;
     }
@@ -45,6 +45,10 @@ for (const compilerEnabled of [false, true]) {
         });
         t.equal(vm.runtime.compilerOptions.enabled, compilerEnabled, 'sanity check');
 
+        vm.runtime.on('COMPILE_ERROR', () => {
+            t.fail('Compile error');
+        });
+
         vm.loadProject(fixture).then(() => {
             let ended = 0;
             vm.runtime.on('SAY', (target, type, text) => {
diff --git a/test/unit/tw_sandboxed_extensions.js b/test/unit/tw_sandboxed_extensions.js
index a763c8b616..ec0fb4b562 100644
--- a/test/unit/tw_sandboxed_extensions.js
+++ b/test/unit/tw_sandboxed_extensions.js
@@ -9,6 +9,9 @@ global.fetch = (url, options = {}) => (
     Promise.resolve(`[Response ${url instanceof Request ? url.url : url} options=${JSON.stringify(options)}]`)
 );
 
+// Remove navigator object from Node 21 and later
+delete global.navigator;
+
 // Need to trick the extension API to think it's running in a worker
 // It will not actually use this object ever.
 global.self = {};
diff --git a/test/unit/tw_translate.js b/test/unit/tw_translate.js
index 8ac5dc7053..3e9c9ccf2c 100644
--- a/test/unit/tw_translate.js
+++ b/test/unit/tw_translate.js
@@ -6,9 +6,12 @@ global.fetch = () => Promise.reject(new Error('Simulated network error'));
 
 const Scratch3TranslateBlocks = require('../../src/extensions/scratch3_translate/index');
 
-global.navigator = {
-    language: 'en'
-};
+// Node 21 and later defines a navigator object, but we want to override that for the test
+Object.defineProperty(global, 'navigator', {
+    value: {
+        language: 'en-US'
+    }
+});
 
 // Translate tries to access AbortController from window, but does not require it to exist.
 global.window = {};
diff --git a/test/unit/tw_unsandboxed_extensions.js b/test/unit/tw_unsandboxed_extensions.js
index a4bfdeec4b..48cf89c12e 100644
--- a/test/unit/tw_unsandboxed_extensions.js
+++ b/test/unit/tw_unsandboxed_extensions.js
@@ -52,6 +52,9 @@ global.window = {
     open: (url, target, features) => `[Window ${url} target=${target || ''} features=${features || ''}]`
 };
 
+// Remove navigator object from Node 21 and later
+delete global.navigator;
+
 tap.beforeEach(() => {
     scriptCallbacks.clear();
     global.location = {
diff --git a/test/unit/util_base64.js b/test/unit/util_base64.js
index c18c353333..f43aa14859 100644
--- a/test/unit/util_base64.js
+++ b/test/unit/util_base64.js
@@ -3,6 +3,7 @@ const Base64Util = require('../../src/util/base64-util');
 
 test('uint8ArrayToBase64', t => {
     t.equal(Base64Util.uint8ArrayToBase64(new Uint8Array([0, 50, 80, 200])), 'ADJQyA==');
+    t.equal(Base64Util.uint8ArrayToBase64([0, 50, 80, 200]), 'ADJQyA==');
     t.end();
 });
 
@@ -25,11 +26,13 @@ test('round trips', t => {
         new Uint8Array(0),
         new Uint8Array([10, 90, 0, 255, 255, 255, 10, 2]),
         new Uint8Array(10000),
-        new Uint8Array(1000000)
+        new Uint8Array(100000)
     ];
     for (const uint8array of data) {
         const uint8ToBase64 = Base64Util.uint8ArrayToBase64(uint8array);
+        const arrayToBase64 = Base64Util.uint8ArrayToBase64(Array.from(uint8array));
         const bufferToBase64 = Base64Util.arrayBufferToBase64(uint8array.buffer);
+        t.equal(uint8ToBase64, arrayToBase64);
         t.equal(uint8ToBase64, bufferToBase64);
         const decoded = Base64Util.base64ToUint8Array(uint8ToBase64);
         t.same(uint8array, decoded);