diff --git a/src/blocks/scratch3_control.js b/src/blocks/scratch3_control.js
index ebf1951514..7324c2d026 100644
--- a/src/blocks/scratch3_control.js
+++ b/src/blocks/scratch3_control.js
@@ -193,13 +193,13 @@ class Scratch3ControlBlocks {
     }
 
     allAtOnce (args, util) {
-        // Since the "all at once" block is implemented for compatiblity with
-        // Scratch 2.0 projects, it behaves the same way it did in 2.0, which
-        // is to simply run the contained script (like "if 1 = 1").
-        // (In early versions of Scratch 2.0, it would work the same way as
-        // "run without screen refresh" custom blocks do now, but this was
-        // removed before the release of 2.0.)
+        // In Scratch 3.0 and TurboWarp, this would simply
+        // run the contained substack. In Unsandboxed,
+        // we've reimplemented the intended functionality
+        // of running the stack all in one frame.
+        util.thread.peekStackFrame().warpMode = false;
         util.startBranch(1, false);
+        util.thread.peekStackFrame().warpMode = true;
     }
 }
 
diff --git a/src/blocks/scratch3_operators.js b/src/blocks/scratch3_operators.js
index a2a5ab4bd2..b730e5706b 100644
--- a/src/blocks/scratch3_operators.js
+++ b/src/blocks/scratch3_operators.js
@@ -25,10 +25,12 @@ class Scratch3OperatorsBlocks {
             operator_gt: this.gt,
             operator_and: this.and,
             operator_or: this.or,
+            operator_xor: this.xor,
             operator_not: this.not,
             operator_random: this.random,
             operator_join: this.join,
             operator_letter_of: this.letterOf,
+            operator_letters_of: this.lettersOf,
             operator_length: this.length,
             operator_contains: this.contains,
             operator_mod: this.mod,
@@ -73,6 +75,10 @@ class Scratch3OperatorsBlocks {
         return Cast.toBoolean(args.OPERAND1) || Cast.toBoolean(args.OPERAND2);
     }
 
+    xor (args) {
+        return Cast.toBoolean(args.OPERAND1) !== Cast.toBoolean(args.OPERAND2);
+    }
+
     not (args) {
         return !Cast.toBoolean(args.OPERAND);
     }
@@ -107,6 +113,12 @@ class Scratch3OperatorsBlocks {
         return str.charAt(index);
     }
 
+    lettersOf (args) {
+        const index1 = Cast.toNumber(args.LETTER1) - 1;
+        const index2 = Cast.toNumber(args.LETTER2) - 1;
+        return str.slice(Math.max(index1, 1), Math.min(str.length, index2));
+    }
+
     length (args) {
         return Cast.toString(args.STRING).length;
     }
diff --git a/src/compiler/irgen.js b/src/compiler/irgen.js
index c3679fb41a..acd32ef2e0 100644
--- a/src/compiler/irgen.js
+++ b/src/compiler/irgen.js
@@ -663,7 +663,7 @@ class ScriptTreeGenerator {
                 const blockInfo = this.getBlockInfo(block.opcode);
                 if (blockInfo) {
                     const type = blockInfo.info.blockType;
-                    if (type === BlockType.REPORTER || type === BlockType.BOOLEAN) {
+                    if (type === BlockType.REPORTER || type === BlockType.BOOLEAN || type === BlockType.INLINE) {
                         return this.descendCompatLayer(block);
                     }
                 }
@@ -694,15 +694,14 @@ class ScriptTreeGenerator {
     descendStackedBlock (block) {
         switch (block.opcode) {
         case 'control_all_at_once':
-            // In Scratch 3, this block behaves like "if 1 = 1"
+            // In Unsandboxed, attempts to run the script in 1 frame.
             return {
-                kind: 'control.if',
+                kind: 'control.allAtOnce',
                 condition: {
                     kind: 'constant',
                     value: true
                 },
-                whenTrue: this.descendSubstack(block, 'SUBSTACK'),
-                whenFalse: []
+                code: this.descendSubstack(block, 'SUBSTACK'),
             };
         case 'control_clear_counter':
             return {
@@ -1415,7 +1414,7 @@ class ScriptTreeGenerator {
         const blockInfo = this.getBlockInfo(block.opcode);
         const blockType = (blockInfo && blockInfo.info && blockInfo.info.blockType) || BlockType.COMMAND;
         const substacks = {};
-        if (blockType === BlockType.CONDITIONAL || blockType === BlockType.LOOP) {
+        if (blockType === BlockType.CONDITIONAL || blockType === BlockType.LOOP || blockType === BlockType.INLINE) {
             for (const inputName in block.inputs) {
                 if (!inputName.startsWith('SUBSTACK')) continue;
                 const branchNum = inputName === 'SUBSTACK' ? 1 : +inputName.substring('SUBSTACK'.length);
diff --git a/src/compiler/jsgen.js b/src/compiler/jsgen.js
index ff07f3f0d5..b9a6c5e538 100644
--- a/src/compiler/jsgen.js
+++ b/src/compiler/jsgen.js
@@ -876,6 +876,13 @@ class JSGenerator {
             this.source += `}\n`;
             break;
 
+        case 'control.allAtOnce':
+            const previousWarp = this.isWarp;
+            this.isWarp = true;
+            this.descendStack(node.code, new Frame(false, 'control.allAtOnce'));
+            this.isWarp = previousWarp;
+            break;
+
         case 'counter.clear':
             this.source += 'runtime.ext_scratch3_control._counter = 0;\n';
             break;
diff --git a/src/engine/runtime.js b/src/engine/runtime.js
index 6fc794268f..d15574f835 100644
--- a/src/engine/runtime.js
+++ b/src/engine/runtime.js
@@ -123,6 +123,10 @@ const ArgumentTypeMap = (() => {
             fieldName: 'SOUND_MENU'
         }
     };
+    map[ArgumentType.VARIABLE] = {
+        fieldType: 'field_variable',
+        fieldName: 'VARIABLE'
+    };
     return map;
 })();
 
@@ -1219,14 +1223,15 @@ class Runtime extends EventEmitter {
                 type: menuId,
                 inputsInline: true,
                 output: 'String',
-                colour: categoryInfo.color1,
-                colourSecondary: categoryInfo.color2,
-                colourTertiary: categoryInfo.color3,
+                colour: menuInfo.acceptText ? '#FFFFFF' : categoryInfo.color1,
+                colourSecondary: menuInfo.acceptText ? '#FFFFFF' : categoryInfo.color2,
+                colourTertiary: menuInfo.acceptText ? '#FFFFFF' : categoryInfo.color3,
                 outputShape: menuInfo.acceptReporters ?
                     ScratchBlocksConstants.OUTPUT_SHAPE_ROUND : ScratchBlocksConstants.OUTPUT_SHAPE_SQUARE,
                 args0: [
-                    {
-                        type: 'field_dropdown',
+                    {// to do: we could reimplement field_numberdropdown here really easily
+                        type: menuInfo.acceptText ?
+                            'field_textdropdown' : 'field_dropdown',
                         name: menuName,
                         options: menuItems
                     }
@@ -1414,6 +1419,11 @@ class Runtime extends EventEmitter {
                 blockJSON.nextStatement = null; // null = available connection; undefined = terminal
             }
             break;
+        case BlockType.INLINE:
+            blockInfo.branchCount = blockInfo.branchCount || 1;
+            blockJSON.output = blockInfo.allowDropAnywhere ? null : 'String'; // TODO: distinguish number & string here?
+            blockJSON.outputShape = ScratchBlocksConstants.OUTPUT_SHAPE_SQUARE;
+            break;
         }
 
         const blockText = Array.isArray(blockInfo.text) ? blockInfo.text : [blockInfo.text];
@@ -1561,7 +1571,7 @@ class Runtime extends EventEmitter {
     }
 
     /**
-     * Helper for _convertPlaceholdes which handles inline images which are a specialized case of block "arguments".
+     * Helper for _convertPlaceholders which handles inline images which are a specialized case of block "arguments".
      * @param {object} argInfo Metadata about the inline image as specified by the extension
      * @return {object} JSON blob for a scratch-blocks image field.
      * @private
@@ -1583,6 +1593,24 @@ class Runtime extends EventEmitter {
         };
     }
 
+    /**
+     * Helper for _convertPlaceholders which handles variable fields which are a specialized case of block "arguments".
+     * @param {object} argInfo Metadata about the inline image as specified by the extension
+     * @return {object} JSON blob for a scratch-blocks variable field.
+     * @private
+     */
+    _constructVariableJson (argInfo, placeholder) {
+        return {
+            type: 'field_variable',
+            name: placeholder,
+            variableTypes:
+                // eslint-disable-next-line max-len
+                argInfo.variableTypes ? (Array.isArray(argInfo.variableTypes) ? argInfo.variableTypes : [argInfo.variableTypes]) : [''],
+            variable: (argInfo.variableTypes === 'broadcast_msg') ? 'message1' : null,
+            filter: argInfo.filter ?? []
+        };
+    }
+
     /**
      * Helper for _convertForScratchBlocks which handles linearization of argument placeholders. Called as a callback
      * from string#replace. In addition to the return value the JSON and XML items in the context will be filled.
@@ -1610,6 +1638,8 @@ class Runtime extends EventEmitter {
         // check if this is not one of those cases. E.g. an inline image on a block.
         if (argTypeInfo.fieldType === 'field_image') {
             argJSON = this._constructInlineImageJson(argInfo);
+        } else if (argTypeInfo.fieldType === 'field_variable') {
+            argJSON = this._constructVariableJson(argInfo, placeholder);
         } else {
             // Construct input value
 
diff --git a/src/extension-support/argument-type.js b/src/extension-support/argument-type.js
index 2e8c13fff8..bf7fbdabb4 100644
--- a/src/extension-support/argument-type.js
+++ b/src/extension-support/argument-type.js
@@ -51,7 +51,12 @@ const ArgumentType = {
     /**
      * Name of sound in the current target
      */
-    SOUND: 'sound'
+    SOUND: 'sound',
+
+    /**
+     * Name of variable in the current specified target(s)
+     */
+    VARIABLE: 'variable'
 };
 
 module.exports = ArgumentType;
diff --git a/src/extension-support/block-type.js b/src/extension-support/block-type.js
index ecaf70754d..4680ef4af1 100644
--- a/src/extension-support/block-type.js
+++ b/src/extension-support/block-type.js
@@ -54,7 +54,13 @@ const BlockType = {
     /**
      * Arbitrary scratch-blocks XML.
      */
-    XML: 'xml'
+    XML: 'xml',
+
+    /**
+     * Specialized reporter block that allows for the insertion and evaluation
+     * of a substack.
+     */
+    INLINE: 'inline'
 };
 
 module.exports = BlockType;
diff --git a/src/extension-support/extension-manager.js b/src/extension-support/extension-manager.js
index 5ec3979e0a..0f48912002 100644
--- a/src/extension-support/extension-manager.js
+++ b/src/extension-support/extension-manager.js
@@ -487,7 +487,8 @@ class ExtensionManager {
             });
 
         if (!menuItems || menuItems.length < 1) {
-            throw new Error(`Extension menu returned no items: ${menuItemFunctionName}`);
+            console.warn(`Extension menu returned no items: ${menuItemFunctionName}`);
+            return [''];
         }
         return menuItems;
     }