diff --git a/core/blockly.js b/core/blockly.js
index 91c643e39..0b6011e07 100644
--- a/core/blockly.js
+++ b/core/blockly.js
@@ -18,6 +18,7 @@ goog.provide('Blockly');
 
 goog.require('Blockly.browserEvents');
 goog.require('Blockly.clipboard');
+goog.require('Blockly.common');
 goog.require('Blockly.ComponentManager');
 goog.require('Blockly.connectionTypes');
 goog.require('Blockly.constants');
@@ -72,12 +73,15 @@ goog.requireType('Blockly.Workspace');
  */
 Blockly.VERSION = 'uncompiled';
 
-/**
- * The main workspace most recently used.
- * Set by Blockly.WorkspaceSvg.prototype.markFocused
- * @type {Blockly.Workspace}
- */
-Blockly.mainWorkspace = null;
+// Add a getter and setter pair for Blockly.mainWorkspace, for legacy reasons.
+Object.defineProperty(Blockly, 'mainWorkspace', {
+  set: function(x) {
+    Blockly.common.setMainWorkspace(x);
+  },
+  get: function() {
+    return Blockly.common.getMainWorkspace();
+  }
+});
 
 /**
  * Currently selected block.
@@ -266,9 +270,7 @@ Blockly.hideChaff = function(opt_onlyClosePopups) {
  * Blockly instances on a page.
  * @return {!Blockly.Workspace} The main workspace.
  */
-Blockly.getMainWorkspace = function() {
-  return /** @type {!Blockly.Workspace} */ (Blockly.mainWorkspace);
-};
+Blockly.getMainWorkspace = Blockly.common.getMainWorkspace;
 
 /**
  * Wrapper to window.alert() that app developers may override to
diff --git a/core/common.js b/core/common.js
new file mode 100644
index 000000000..d9a6b9638
--- /dev/null
+++ b/core/common.js
@@ -0,0 +1,46 @@
+/**
+ * @license
+ * Copyright 2021 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Common functions used both internally and externally, but which
+ * must not be at the top level to avoid circular dependencies.
+ * @author fenichel@google.com (Rachel Fenichel)
+ */
+'use strict';
+
+goog.module('Blockly.common');
+goog.module.declareLegacyNamespace();
+
+/* eslint-disable-next-line no-unused-vars */
+const Workspace = goog.requireType('Blockly.Workspace');
+
+
+/**
+ * The main workspace most recently used.
+ * Set by Blockly.WorkspaceSvg.prototype.markFocused
+ * @type {!Workspace}
+ */
+let mainWorkspace;
+
+/**
+ * Returns the last used top level workspace (based on focus).  Try not to use
+ * this function, particularly if there are multiple Blockly instances on a
+ * page.
+ * @return {!Workspace} The main workspace.
+ */
+const getMainWorkspace = function() {
+  return mainWorkspace;
+};
+exports.getMainWorkspace = getMainWorkspace;
+
+/**
+ * Sets last used main workspace.
+ * @param {!Workspace} workspace The most recently used top level workspace.
+ */
+const setMainWorkspace = function(workspace) {
+  mainWorkspace = workspace;
+};
+exports.setMainWorkspace = setMainWorkspace;
diff --git a/tests/deps.js b/tests/deps.js
index b0c73600a..c83f1782a 100644
--- a/tests/deps.js
+++ b/tests/deps.js
@@ -12,13 +12,18 @@ goog.addDependency('../../core/block_animations.js', ['Blockly.blockAnimations']
 goog.addDependency('../../core/block_drag_surface.js', ['Blockly.BlockDragSurfaceSvg'], ['Blockly.utils', 'Blockly.utils.Coordinate', 'Blockly.utils.Svg', 'Blockly.utils.dom'], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/block_dragger.js', ['Blockly.BlockDragger'], ['Blockly.Events', 'Blockly.Events.BlockDrag', 'Blockly.Events.BlockMove', 'Blockly.IBlockDragger', 'Blockly.InsertionMarkerManager', 'Blockly.blockAnimations', 'Blockly.registry', 'Blockly.utils.Coordinate', 'Blockly.utils.dom'], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/block_svg.js', ['Blockly.BlockSvg'], ['Blockly.ASTNode', 'Blockly.Block', 'Blockly.ContextMenu', 'Blockly.ContextMenuRegistry', 'Blockly.Events', 'Blockly.Events.BlockMove', 'Blockly.Events.Selected', 'Blockly.IASTNodeLocationSvg', 'Blockly.IBoundedElement', 'Blockly.ICopyable', 'Blockly.IDraggable', 'Blockly.Msg', 'Blockly.RenderedConnection', 'Blockly.TabNavigateCursor', 'Blockly.Tooltip', 'Blockly.Touch', 'Blockly.Xml', 'Blockly.blockAnimations', 'Blockly.blockRendering.IPathObject', 'Blockly.browserEvents', 'Blockly.connectionTypes', 'Blockly.constants', 'Blockly.internalConstants', 'Blockly.utils', 'Blockly.utils.Coordinate', 'Blockly.utils.Rect', 'Blockly.utils.Svg', 'Blockly.utils.deprecation', 'Blockly.utils.dom', 'Blockly.utils.object', 'Blockly.utils.userAgent']);
+<<<<<<< HEAD
 goog.addDependency('../../core/blockly.js', ['Blockly'], ['Blockly.ComponentManager', 'Blockly.DropDownDiv', 'Blockly.Events', 'Blockly.Events.BlockCreate', 'Blockly.Events.FinishedLoading', 'Blockly.Events.Ui', 'Blockly.Events.UiBase', 'Blockly.Events.VarCreate', 'Blockly.Procedures', 'Blockly.ShortcutRegistry', 'Blockly.Tooltip', 'Blockly.Touch', 'Blockly.Variables', 'Blockly.WidgetDiv', 'Blockly.WorkspaceSvg', 'Blockly.Xml', 'Blockly.browserEvents', 'Blockly.clipboard', 'Blockly.connectionTypes', 'Blockly.constants', 'Blockly.inject', 'Blockly.inputTypes', 'Blockly.internalConstants', 'Blockly.utils', 'Blockly.utils.Size', 'Blockly.utils.colour', 'Blockly.utils.deprecation', 'Blockly.utils.toolbox']);
+=======
+goog.addDependency('../../core/blockly.js', ['Blockly'], ['Blockly.ComponentManager', 'Blockly.DropDownDiv', 'Blockly.Events', 'Blockly.Events.BlockCreate', 'Blockly.Events.FinishedLoading', 'Blockly.Events.Ui', 'Blockly.Events.UiBase', 'Blockly.Events.VarCreate', 'Blockly.Procedures', 'Blockly.ShortcutRegistry', 'Blockly.Tooltip', 'Blockly.Touch', 'Blockly.Variables', 'Blockly.WidgetDiv', 'Blockly.WorkspaceSvg', 'Blockly.Xml', 'Blockly.browserEvents', 'Blockly.common', 'Blockly.connectionTypes', 'Blockly.constants', 'Blockly.inject', 'Blockly.inputTypes', 'Blockly.internalConstants', 'Blockly.utils', 'Blockly.utils.Size', 'Blockly.utils.colour', 'Blockly.utils.deprecation', 'Blockly.utils.toolbox']);
+>>>>>>> 2c0751ed (Move main workspace to a new file, common.js)
 goog.addDependency('../../core/blocks.js', ['Blockly.Blocks'], [], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/browser_events.js', ['Blockly.browserEvents'], ['Blockly.Touch', 'Blockly.utils.global']);
 goog.addDependency('../../core/bubble.js', ['Blockly.Bubble'], ['Blockly.IBubble', 'Blockly.Scrollbar', 'Blockly.Touch', 'Blockly.Workspace', 'Blockly.browserEvents', 'Blockly.utils', 'Blockly.utils.Coordinate', 'Blockly.utils.Size', 'Blockly.utils.Svg', 'Blockly.utils.dom', 'Blockly.utils.math', 'Blockly.utils.userAgent']);
 goog.addDependency('../../core/bubble_dragger.js', ['Blockly.BubbleDragger'], ['Blockly.Bubble', 'Blockly.ComponentManager', 'Blockly.Events', 'Blockly.Events.CommentMove', 'Blockly.constants', 'Blockly.utils', 'Blockly.utils.Coordinate'], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/clipboard.js', ['Blockly.clipboard'], ['Blockly.Events'], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/comment.js', ['Blockly.Comment'], ['Blockly.Bubble', 'Blockly.Css', 'Blockly.Events', 'Blockly.Events.BlockChange', 'Blockly.Events.BubbleOpen', 'Blockly.Icon', 'Blockly.Warning', 'Blockly.browserEvents', 'Blockly.utils.Svg', 'Blockly.utils.dom', 'Blockly.utils.object', 'Blockly.utils.userAgent'], {'lang': 'es6', 'module': 'goog'});
+goog.addDependency('../../core/common.js', ['Blockly.common'], [], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/component_manager.js', ['Blockly.ComponentManager'], [], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/connection.js', ['Blockly.Connection'], ['Blockly.Events', 'Blockly.Events.BlockMove', 'Blockly.IASTNodeLocationWithBlock', 'Blockly.Xml', 'Blockly.connectionTypes', 'Blockly.constants', 'Blockly.utils.deprecation'], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/connection_checker.js', ['Blockly.ConnectionChecker'], ['Blockly.Connection', 'Blockly.IConnectionChecker', 'Blockly.connectionTypes', 'Blockly.internalConstants', 'Blockly.registry'], {'lang': 'es6', 'module': 'goog'});