From 26a029d407be480d791972afb5975cf62c9360a6 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Fri, 19 Apr 2024 02:47:55 +0200
Subject: Adding upstream version 124.0.1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 .../client/inspector/changes/ChangesContextMenu.js | 110 ++++++
 devtools/client/inspector/changes/ChangesView.js   | 284 +++++++++++++++
 .../client/inspector/changes/actions/changes.js    |  25 ++
 devtools/client/inspector/changes/actions/index.js |  18 +
 .../client/inspector/changes/actions/moz.build     |  10 +
 .../inspector/changes/components/CSSDeclaration.js |  47 +++
 .../inspector/changes/components/ChangesApp.js     | 241 +++++++++++++
 .../client/inspector/changes/components/moz.build  |  10 +
 devtools/client/inspector/changes/moz.build        |  24 ++
 .../client/inspector/changes/reducers/changes.js   | 385 +++++++++++++++++++++
 .../client/inspector/changes/reducers/moz.build    |   9 +
 .../client/inspector/changes/selectors/changes.js  | 261 ++++++++++++++
 .../client/inspector/changes/selectors/moz.build   |   9 +
 .../client/inspector/changes/test/browser.toml     |  45 +++
 .../test/browser_changes_background_tracking.js    |  46 +++
 .../test/browser_changes_copy_all_changes.js       |  53 +++
 .../test/browser_changes_copy_declaration.js       |  67 ++++
 .../changes/test/browser_changes_copy_rule.js      |  64 ++++
 ...er_changes_declaration_add_special_character.js |  78 +++++
 .../test/browser_changes_declaration_disable.js    |  48 +++
 .../test/browser_changes_declaration_duplicate.js  | 107 ++++++
 .../test/browser_changes_declaration_edit_value.js | 170 +++++++++
 .../browser_changes_declaration_identical_rules.js |  71 ++++
 .../test/browser_changes_declaration_remove.js     |  43 +++
 .../browser_changes_declaration_remove_ahead.js    |  53 +++
 .../browser_changes_declaration_remove_disabled.js | 107 ++++++
 .../test/browser_changes_declaration_rename.js     |  68 ++++
 .../changes/test/browser_changes_nested_rules.js   | 189 ++++++++++
 .../changes/test/browser_changes_rule_add.js       |  64 ++++
 .../changes/test/browser_changes_rule_selector.js  |  60 ++++
 devtools/client/inspector/changes/test/head.js     |  93 +++++
 .../inspector/changes/test/xpcshell/.eslintrc.js   |   6 +
 .../client/inspector/changes/test/xpcshell/head.js |   8 +
 .../inspector/changes/test/xpcshell/mocks.js       |  67 ++++
 .../test/xpcshell/test_changes_stylesheet.js       |  60 ++++
 .../inspector/changes/test/xpcshell/xpcshell.toml  |   7 +
 .../inspector/changes/utils/changes-utils.js       |  44 +++
 devtools/client/inspector/changes/utils/l10n.js    |  15 +
 devtools/client/inspector/changes/utils/moz.build  |  10 +
 39 files changed, 3076 insertions(+)
 create mode 100644 devtools/client/inspector/changes/ChangesContextMenu.js
 create mode 100644 devtools/client/inspector/changes/ChangesView.js
 create mode 100644 devtools/client/inspector/changes/actions/changes.js
 create mode 100644 devtools/client/inspector/changes/actions/index.js
 create mode 100644 devtools/client/inspector/changes/actions/moz.build
 create mode 100644 devtools/client/inspector/changes/components/CSSDeclaration.js
 create mode 100644 devtools/client/inspector/changes/components/ChangesApp.js
 create mode 100644 devtools/client/inspector/changes/components/moz.build
 create mode 100644 devtools/client/inspector/changes/moz.build
 create mode 100644 devtools/client/inspector/changes/reducers/changes.js
 create mode 100644 devtools/client/inspector/changes/reducers/moz.build
 create mode 100644 devtools/client/inspector/changes/selectors/changes.js
 create mode 100644 devtools/client/inspector/changes/selectors/moz.build
 create mode 100644 devtools/client/inspector/changes/test/browser.toml
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_background_tracking.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_copy_all_changes.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_copy_declaration.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_copy_rule.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_declaration_add_special_character.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_declaration_disable.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_declaration_duplicate.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_declaration_edit_value.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_declaration_identical_rules.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_declaration_remove.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_declaration_remove_ahead.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_declaration_remove_disabled.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_declaration_rename.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_nested_rules.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_rule_add.js
 create mode 100644 devtools/client/inspector/changes/test/browser_changes_rule_selector.js
 create mode 100644 devtools/client/inspector/changes/test/head.js
 create mode 100644 devtools/client/inspector/changes/test/xpcshell/.eslintrc.js
 create mode 100644 devtools/client/inspector/changes/test/xpcshell/head.js
 create mode 100644 devtools/client/inspector/changes/test/xpcshell/mocks.js
 create mode 100644 devtools/client/inspector/changes/test/xpcshell/test_changes_stylesheet.js
 create mode 100644 devtools/client/inspector/changes/test/xpcshell/xpcshell.toml
 create mode 100644 devtools/client/inspector/changes/utils/changes-utils.js
 create mode 100644 devtools/client/inspector/changes/utils/l10n.js
 create mode 100644 devtools/client/inspector/changes/utils/moz.build

(limited to 'devtools/client/inspector/changes')

diff --git a/devtools/client/inspector/changes/ChangesContextMenu.js b/devtools/client/inspector/changes/ChangesContextMenu.js
new file mode 100644
index 0000000000..40257ec898
--- /dev/null
+++ b/devtools/client/inspector/changes/ChangesContextMenu.js
@@ -0,0 +1,110 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const Menu = require("resource://devtools/client/framework/menu.js");
+loader.lazyRequireGetter(
+  this,
+  "MenuItem",
+  "resource://devtools/client/framework/menu-item.js"
+);
+
+const {
+  getStr,
+} = require("resource://devtools/client/inspector/changes/utils/l10n.js");
+
+/**
+ * Context menu for the Changes panel with options to select, copy and export CSS changes.
+ */
+class ChangesContextMenu extends Menu {
+  constructor(config = {}) {
+    super(config);
+    this.onCopy = config.onCopy;
+    this.onCopyAllChanges = config.onCopyAllChanges;
+    this.onCopyDeclaration = config.onCopyDeclaration;
+    this.onCopyRule = config.onCopyRule;
+    this.onSelectAll = config.onSelectAll;
+    this.toolboxDocument = config.toolboxDocument;
+    this.window = config.window;
+  }
+
+  show(event) {
+    this._openMenu({
+      target: event.target,
+      screenX: event.screenX,
+      screenY: event.screenY,
+    });
+  }
+
+  _openMenu({ target, screenX = 0, screenY = 0 } = {}) {
+    this.window.focus();
+    // Remove existing menu items.
+    this.clear();
+
+    // Copy option
+    const menuitemCopy = new MenuItem({
+      id: "changes-contextmenu-copy",
+      label: getStr("changes.contextmenu.copy"),
+      accesskey: getStr("changes.contextmenu.copy.accessKey"),
+      click: this.onCopy,
+      disabled: !this._hasTextSelected(),
+    });
+    this.append(menuitemCopy);
+
+    const declEl = target.closest(".changes__declaration");
+    const ruleEl = target.closest("[data-rule-id]");
+    const ruleId = ruleEl ? ruleEl.dataset.ruleId : null;
+
+    if (ruleId || declEl) {
+      // Copy Rule option
+      this.append(
+        new MenuItem({
+          id: "changes-contextmenu-copy-rule",
+          label: getStr("changes.contextmenu.copyRule"),
+          click: () => this.onCopyRule(ruleId, true),
+        })
+      );
+
+      // Copy Declaration option. Visible only if there is a declaration element target.
+      this.append(
+        new MenuItem({
+          id: "changes-contextmenu-copy-declaration",
+          label: getStr("changes.contextmenu.copyDeclaration"),
+          click: () => this.onCopyDeclaration(declEl),
+          visible: !!declEl,
+        })
+      );
+
+      this.append(
+        new MenuItem({
+          type: "separator",
+        })
+      );
+    }
+
+    // Select All option
+    const menuitemSelectAll = new MenuItem({
+      id: "changes-contextmenu-select-all",
+      label: getStr("changes.contextmenu.selectAll"),
+      accesskey: getStr("changes.contextmenu.selectAll.accessKey"),
+      click: this.onSelectAll,
+    });
+    this.append(menuitemSelectAll);
+
+    this.popup(screenX, screenY, this.toolboxDocument);
+  }
+
+  _hasTextSelected() {
+    const selection = this.window.getSelection();
+    return selection.toString() && !selection.isCollapsed;
+  }
+
+  destroy() {
+    this.window = null;
+    this.toolboxDocument = null;
+  }
+}
+
+module.exports = ChangesContextMenu;
diff --git a/devtools/client/inspector/changes/ChangesView.js b/devtools/client/inspector/changes/ChangesView.js
new file mode 100644
index 0000000000..20c70137a7
--- /dev/null
+++ b/devtools/client/inspector/changes/ChangesView.js
@@ -0,0 +1,284 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  createFactory,
+  createElement,
+} = require("resource://devtools/client/shared/vendor/react.js");
+const {
+  Provider,
+} = require("resource://devtools/client/shared/vendor/react-redux.js");
+
+loader.lazyRequireGetter(
+  this,
+  "ChangesContextMenu",
+  "resource://devtools/client/inspector/changes/ChangesContextMenu.js"
+);
+loader.lazyRequireGetter(
+  this,
+  "clipboardHelper",
+  "resource://devtools/shared/platform/clipboard.js"
+);
+
+const changesReducer = require("resource://devtools/client/inspector/changes/reducers/changes.js");
+const {
+  getChangesStylesheet,
+} = require("resource://devtools/client/inspector/changes/selectors/changes.js");
+const {
+  resetChanges,
+  trackChange,
+} = require("resource://devtools/client/inspector/changes/actions/changes.js");
+
+const ChangesApp = createFactory(
+  require("resource://devtools/client/inspector/changes/components/ChangesApp.js")
+);
+
+class ChangesView {
+  constructor(inspector, window) {
+    this.document = window.document;
+    this.inspector = inspector;
+    this.store = this.inspector.store;
+    this.telemetry = this.inspector.telemetry;
+    this.window = window;
+
+    this.store.injectReducer("changes", changesReducer);
+
+    this.onAddChange = this.onAddChange.bind(this);
+    this.onContextMenu = this.onContextMenu.bind(this);
+    this.onCopy = this.onCopy.bind(this);
+    this.onCopyAllChanges = this.copyAllChanges.bind(this);
+    this.onCopyDeclaration = this.copyDeclaration.bind(this);
+    this.onCopyRule = this.copyRule.bind(this);
+    this.onClearChanges = this.onClearChanges.bind(this);
+    this.onSelectAll = this.onSelectAll.bind(this);
+    this.onResourceAvailable = this.onResourceAvailable.bind(this);
+
+    this.destroy = this.destroy.bind(this);
+
+    this.init();
+  }
+
+  get contextMenu() {
+    if (!this._contextMenu) {
+      this._contextMenu = new ChangesContextMenu({
+        onCopy: this.onCopy,
+        onCopyAllChanges: this.onCopyAllChanges,
+        onCopyDeclaration: this.onCopyDeclaration,
+        onCopyRule: this.onCopyRule,
+        onSelectAll: this.onSelectAll,
+        toolboxDocument: this.inspector.toolbox.doc,
+        window: this.window,
+      });
+    }
+
+    return this._contextMenu;
+  }
+
+  get resourceCommand() {
+    return this.inspector.toolbox.resourceCommand;
+  }
+
+  init() {
+    const changesApp = ChangesApp({
+      onContextMenu: this.onContextMenu,
+      onCopyAllChanges: this.onCopyAllChanges,
+      onCopyRule: this.onCopyRule,
+    });
+
+    // Expose the provider to let inspector.js use it in setupSidebar.
+    this.provider = createElement(
+      Provider,
+      {
+        id: "changesview",
+        key: "changesview",
+        store: this.store,
+      },
+      changesApp
+    );
+
+    this.watchResources();
+  }
+
+  async watchResources() {
+    await this.resourceCommand.watchResources(
+      [this.resourceCommand.TYPES.DOCUMENT_EVENT],
+      {
+        onAvailable: this.onResourceAvailable,
+        // Ignore any DOCUMENT_EVENT resources that have occured in the past
+        // and are cached by the resource command, otherwise the Changes panel will
+        // react to them erroneously and interpret that the document is reloading *now*
+        // which leads to clearing all stored changes.
+        ignoreExistingResources: true,
+      }
+    );
+
+    await this.resourceCommand.watchResources(
+      [this.resourceCommand.TYPES.CSS_CHANGE],
+      { onAvailable: this.onResourceAvailable }
+    );
+  }
+
+  onResourceAvailable(resources) {
+    for (const resource of resources) {
+      if (resource.resourceType === this.resourceCommand.TYPES.CSS_CHANGE) {
+        this.onAddChange(resource);
+        continue;
+      }
+
+      if (resource.name === "dom-loading" && resource.targetFront.isTopLevel) {
+        // will-navigate doesn't work when we navigate to a new process,
+        // and for now, onTargetAvailable/onTargetDestroyed doesn't fire on navigation and
+        // only when navigating to another process.
+        // So we fallback on DOCUMENT_EVENTS to be notified when we navigate. When we
+        // navigate within the same process as well as when we navigate to a new process.
+        // (We would probably revisit that in bug 1632141)
+        this.onClearChanges();
+      }
+    }
+  }
+
+  /**
+   * Handler for the "Copy All Changes" button. Simple wrapper that just calls
+   * |this.copyChanges()| with no filters in order to trigger default operation.
+   */
+  copyAllChanges() {
+    this.copyChanges();
+  }
+
+  /**
+   * Handler for the "Copy Changes" option from the context menu.
+   * Builds a CSS text with the aggregated changes and copies it to the clipboard.
+   *
+   * Optional rule and source ids can be used to filter the scope of the operation:
+   * - if both a rule id and source id are provided, copy only the changes to the
+   * matching rule within the matching source.
+   * - if only a source id is provided, copy the changes to all rules within the
+   * matching source.
+   * - if neither rule id nor source id are provided, copy the changes too all rules
+   * within all sources.
+   *
+   * @param {String|null} ruleId
+   *        Optional rule id.
+   * @param {String|null} sourceId
+   *        Optional source id.
+   */
+  copyChanges(ruleId, sourceId) {
+    const state = this.store.getState().changes || {};
+    const filter = {};
+    if (ruleId) {
+      filter.ruleIds = [ruleId];
+    }
+    if (sourceId) {
+      filter.sourceIds = [sourceId];
+    }
+
+    const text = getChangesStylesheet(state, filter);
+    clipboardHelper.copyString(text);
+  }
+
+  /**
+   * Handler for the "Copy Declaration" option from the context menu.
+   * Builds a CSS declaration string with the property name and value, and copies it
+   * to the clipboard. The declaration is commented out if it is marked as removed.
+   *
+   * @param {DOMElement} element
+   *        Host element of a CSS declaration rendered the Changes panel.
+   */
+  copyDeclaration(element) {
+    const name = element.querySelector(
+      ".changes__declaration-name"
+    ).textContent;
+    const value = element.querySelector(
+      ".changes__declaration-value"
+    ).textContent;
+    const isRemoved = element.classList.contains("diff-remove");
+    const text = isRemoved ? `/* ${name}: ${value}; */` : `${name}: ${value};`;
+    clipboardHelper.copyString(text);
+  }
+
+  /**
+   * Handler for the "Copy Rule" option from the context menu and "Copy Rule" button.
+   * Gets the full content of the target CSS rule (including any changes applied)
+   * and copies it to the clipboard.
+   *
+   * @param {String} ruleId
+   *        Rule id of the target CSS rule.
+   */
+  async copyRule(ruleId) {
+    const inspectorFronts = await this.inspector.getAllInspectorFronts();
+
+    for (const inspectorFront of inspectorFronts) {
+      const rule = await inspectorFront.pageStyle.getRule(ruleId);
+
+      if (rule) {
+        const text = await rule.getRuleText();
+        clipboardHelper.copyString(text);
+        break;
+      }
+    }
+  }
+
+  /**
+   * Handler for the "Copy" option from the context menu.
+   * Copies the current text selection to the clipboard.
+   */
+  onCopy() {
+    clipboardHelper.copyString(this.window.getSelection().toString());
+  }
+
+  onAddChange(change) {
+    // Turn data into a suitable change to send to the store.
+    this.store.dispatch(trackChange(change));
+  }
+
+  onClearChanges() {
+    this.store.dispatch(resetChanges());
+  }
+
+  /**
+   * Select all text.
+   */
+  onSelectAll() {
+    const selection = this.window.getSelection();
+    selection.selectAllChildren(
+      this.document.getElementById("sidebar-panel-changes")
+    );
+  }
+
+  /**
+   * Event handler for the "contextmenu" event fired when the context menu is requested.
+   * @param {Event} e
+   */
+  onContextMenu(e) {
+    this.contextMenu.show(e);
+  }
+
+  /**
+   * Destruction function called when the inspector is destroyed.
+   */
+  destroy() {
+    this.resourceCommand.unwatchResources(
+      [
+        this.resourceCommand.TYPES.CSS_CHANGE,
+        this.resourceCommand.TYPES.DOCUMENT_EVENT,
+      ],
+      { onAvailable: this.onResourceAvailable }
+    );
+
+    this.store.dispatch(resetChanges());
+
+    this.document = null;
+    this.inspector = null;
+    this.store = null;
+
+    if (this._contextMenu) {
+      this._contextMenu.destroy();
+      this._contextMenu = null;
+    }
+  }
+}
+
+module.exports = ChangesView;
diff --git a/devtools/client/inspector/changes/actions/changes.js b/devtools/client/inspector/changes/actions/changes.js
new file mode 100644
index 0000000000..b3b02cfa2a
--- /dev/null
+++ b/devtools/client/inspector/changes/actions/changes.js
@@ -0,0 +1,25 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  RESET_CHANGES,
+  TRACK_CHANGE,
+} = require("resource://devtools/client/inspector/changes/actions/index.js");
+
+module.exports = {
+  resetChanges() {
+    return {
+      type: RESET_CHANGES,
+    };
+  },
+
+  trackChange(change) {
+    return {
+      type: TRACK_CHANGE,
+      change,
+    };
+  },
+};
diff --git a/devtools/client/inspector/changes/actions/index.js b/devtools/client/inspector/changes/actions/index.js
new file mode 100644
index 0000000000..3ae33883f2
--- /dev/null
+++ b/devtools/client/inspector/changes/actions/index.js
@@ -0,0 +1,18 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const { createEnum } = require("resource://devtools/client/shared/enum.js");
+
+createEnum(
+  [
+    // Remove all changes
+    "RESET_CHANGES",
+
+    // Track a style change
+    "TRACK_CHANGE",
+  ],
+  module.exports
+);
diff --git a/devtools/client/inspector/changes/actions/moz.build b/devtools/client/inspector/changes/actions/moz.build
new file mode 100644
index 0000000000..06c5314a9e
--- /dev/null
+++ b/devtools/client/inspector/changes/actions/moz.build
@@ -0,0 +1,10 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+DevToolsModules(
+    "changes.js",
+    "index.js",
+)
diff --git a/devtools/client/inspector/changes/components/CSSDeclaration.js b/devtools/client/inspector/changes/components/CSSDeclaration.js
new file mode 100644
index 0000000000..c25bf6833f
--- /dev/null
+++ b/devtools/client/inspector/changes/components/CSSDeclaration.js
@@ -0,0 +1,47 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  PureComponent,
+} = require("resource://devtools/client/shared/vendor/react.js");
+const dom = require("resource://devtools/client/shared/vendor/react-dom-factories.js");
+const PropTypes = require("resource://devtools/client/shared/vendor/react-prop-types.js");
+
+class CSSDeclaration extends PureComponent {
+  static get propTypes() {
+    return {
+      className: PropTypes.string,
+      property: PropTypes.string.isRequired,
+      value: PropTypes.string.isRequired,
+    };
+  }
+
+  static get defaultProps() {
+    return {
+      className: "",
+    };
+  }
+
+  render() {
+    const { className, property, value } = this.props;
+
+    return dom.div(
+      { className: `changes__declaration ${className}` },
+      dom.span(
+        { className: "changes__declaration-name theme-fg-color3" },
+        property
+      ),
+      ": ",
+      dom.span(
+        { className: "changes__declaration-value theme-fg-color1" },
+        value
+      ),
+      ";"
+    );
+  }
+}
+
+module.exports = CSSDeclaration;
diff --git a/devtools/client/inspector/changes/components/ChangesApp.js b/devtools/client/inspector/changes/components/ChangesApp.js
new file mode 100644
index 0000000000..9953109e19
--- /dev/null
+++ b/devtools/client/inspector/changes/components/ChangesApp.js
@@ -0,0 +1,241 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  createFactory,
+  PureComponent,
+} = require("resource://devtools/client/shared/vendor/react.js");
+const dom = require("resource://devtools/client/shared/vendor/react-dom-factories.js");
+const PropTypes = require("resource://devtools/client/shared/vendor/react-prop-types.js");
+const {
+  connect,
+} = require("resource://devtools/client/shared/vendor/react-redux.js");
+
+const CSSDeclaration = createFactory(
+  require("resource://devtools/client/inspector/changes/components/CSSDeclaration.js")
+);
+const {
+  getChangesTree,
+} = require("resource://devtools/client/inspector/changes/selectors/changes.js");
+const {
+  getSourceForDisplay,
+} = require("resource://devtools/client/inspector/changes/utils/changes-utils.js");
+const {
+  getStr,
+} = require("resource://devtools/client/inspector/changes/utils/l10n.js");
+
+class ChangesApp extends PureComponent {
+  static get propTypes() {
+    return {
+      // Nested CSS rule tree structure of CSS changes grouped by source (stylesheet)
+      changesTree: PropTypes.object.isRequired,
+      // Event handler for "contextmenu" event
+      onContextMenu: PropTypes.func.isRequired,
+      // Event handler for click on "Copy All Changes" button
+      onCopyAllChanges: PropTypes.func.isRequired,
+      // Event handler for click on "Copy Rule" button
+      onCopyRule: PropTypes.func.isRequired,
+    };
+  }
+
+  constructor(props) {
+    super(props);
+  }
+
+  renderCopyAllChangesButton() {
+    const button = dom.button(
+      {
+        className: "changes__copy-all-changes-button",
+        onClick: e => {
+          e.stopPropagation();
+          this.props.onCopyAllChanges();
+        },
+        title: getStr("changes.contextmenu.copyAllChangesDescription"),
+      },
+      getStr("changes.contextmenu.copyAllChanges")
+    );
+
+    return dom.div({ className: "changes__toolbar" }, button);
+  }
+
+  renderCopyButton(ruleId) {
+    return dom.button(
+      {
+        className: "changes__copy-rule-button",
+        onClick: e => {
+          e.stopPropagation();
+          this.props.onCopyRule(ruleId);
+        },
+        title: getStr("changes.contextmenu.copyRuleDescription"),
+      },
+      getStr("changes.contextmenu.copyRule")
+    );
+  }
+
+  renderDeclarations(remove = [], add = []) {
+    const removals = remove
+      // Sorting changed declarations in the order they appear in the Rules view.
+      .sort((a, b) => a.index > b.index)
+      .map(({ property, value, index }) => {
+        return CSSDeclaration({
+          key: "remove-" + property + index,
+          className: "level diff-remove",
+          property,
+          value,
+        });
+      });
+
+    const additions = add
+      // Sorting changed declarations in the order they appear in the Rules view.
+      .sort((a, b) => a.index > b.index)
+      .map(({ property, value, index }) => {
+        return CSSDeclaration({
+          key: "add-" + property + index,
+          className: "level diff-add",
+          property,
+          value,
+        });
+      });
+
+    return [removals, additions];
+  }
+
+  renderRule(ruleId, rule, level = 0) {
+    const diffClass = rule.isNew ? "diff-add" : "";
+    return dom.div(
+      {
+        key: ruleId,
+        className: "changes__rule devtools-monospace",
+        "data-rule-id": ruleId,
+        style: {
+          "--diff-level": level,
+        },
+      },
+      this.renderSelectors(rule.selectors, rule.isNew),
+      this.renderCopyButton(ruleId),
+      // Render any nested child rules if they exist.
+      rule.children.map(childRule => {
+        return this.renderRule(childRule.ruleId, childRule, level + 1);
+      }),
+      // Render any changed CSS declarations.
+      this.renderDeclarations(rule.remove, rule.add),
+      // Render the closing bracket with a diff marker if necessary.
+      dom.div({ className: `level ${diffClass}` }, "}")
+    );
+  }
+
+  /**
+   * Return an array of React elements for the rule's selector.
+   *
+   * @param  {Array} selectors
+   *         List of strings as versions of this rule's selector over time.
+   * @param  {Boolean} isNewRule
+   *         Whether the rule was created at runtime.
+   * @return {Array}
+   */
+  renderSelectors(selectors, isNewRule) {
+    const selectorDiffClassMap = new Map();
+
+    // The selectors array has just one item if it hasn't changed. Render it as-is.
+    // If the rule was created at runtime, mark the single selector as added.
+    // If it has two or more items, the first item was the original selector (mark as
+    // removed) and the last item is the current selector (mark as added).
+    if (selectors.length === 1) {
+      selectorDiffClassMap.set(selectors[0], isNewRule ? "diff-add" : "");
+    } else if (selectors.length >= 2) {
+      selectorDiffClassMap.set(selectors[0], "diff-remove");
+      selectorDiffClassMap.set(selectors[selectors.length - 1], "diff-add");
+    }
+
+    const elements = [];
+
+    for (const [selector, diffClass] of selectorDiffClassMap) {
+      elements.push(
+        dom.div(
+          {
+            key: selector,
+            className: `level changes__selector ${diffClass}`,
+            title: selector,
+          },
+          selector,
+          dom.span({}, " {")
+        )
+      );
+    }
+
+    return elements;
+  }
+
+  renderDiff(changes = {}) {
+    // Render groups of style sources: stylesheets and element style attributes.
+    return Object.entries(changes).map(([sourceId, source]) => {
+      const path = getSourceForDisplay(source);
+      const { href, rules, isFramed } = source;
+
+      return dom.div(
+        {
+          key: sourceId,
+          "data-source-id": sourceId,
+          className: "source",
+        },
+        dom.div(
+          {
+            className: "href",
+            title: href,
+          },
+          dom.span({}, path),
+          isFramed && this.renderFrameBadge(href)
+        ),
+        // Render changed rules within this source.
+        Object.entries(rules).map(([ruleId, rule]) => {
+          return this.renderRule(ruleId, rule);
+        })
+      );
+    });
+  }
+
+  renderFrameBadge(href = "") {
+    return dom.span(
+      {
+        className: "inspector-badge",
+        title: href,
+      },
+      getStr("changes.iframeLabel")
+    );
+  }
+
+  renderEmptyState() {
+    return dom.div(
+      { className: "devtools-sidepanel-no-result" },
+      dom.p({}, getStr("changes.noChanges")),
+      dom.p({}, getStr("changes.noChangesDescription"))
+    );
+  }
+
+  render() {
+    const hasChanges = !!Object.keys(this.props.changesTree).length;
+    return dom.div(
+      {
+        className: "theme-sidebar inspector-tabpanel",
+        id: "sidebar-panel-changes",
+        role: "document",
+        tabIndex: "0",
+        onContextMenu: this.props.onContextMenu,
+      },
+      !hasChanges && this.renderEmptyState(),
+      hasChanges && this.renderCopyAllChangesButton(),
+      hasChanges && this.renderDiff(this.props.changesTree)
+    );
+  }
+}
+
+const mapStateToProps = state => {
+  return {
+    changesTree: getChangesTree(state.changes),
+  };
+};
+
+module.exports = connect(mapStateToProps)(ChangesApp);
diff --git a/devtools/client/inspector/changes/components/moz.build b/devtools/client/inspector/changes/components/moz.build
new file mode 100644
index 0000000000..e8fba36fb8
--- /dev/null
+++ b/devtools/client/inspector/changes/components/moz.build
@@ -0,0 +1,10 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+DevToolsModules(
+    "ChangesApp.js",
+    "CSSDeclaration.js",
+)
diff --git a/devtools/client/inspector/changes/moz.build b/devtools/client/inspector/changes/moz.build
new file mode 100644
index 0000000000..7d504f13f5
--- /dev/null
+++ b/devtools/client/inspector/changes/moz.build
@@ -0,0 +1,24 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+DIRS += [
+    "actions",
+    "components",
+    "reducers",
+    "selectors",
+    "utils",
+]
+
+DevToolsModules(
+    "ChangesContextMenu.js",
+    "ChangesView.js",
+)
+
+BROWSER_CHROME_MANIFESTS += ["test/browser.toml"]
+XPCSHELL_TESTS_MANIFESTS += ["test/xpcshell/xpcshell.toml"]
+
+with Files("**"):
+    BUG_COMPONENT = ("DevTools", "Inspector: Changes")
diff --git a/devtools/client/inspector/changes/reducers/changes.js b/devtools/client/inspector/changes/reducers/changes.js
new file mode 100644
index 0000000000..23e82a3ba7
--- /dev/null
+++ b/devtools/client/inspector/changes/reducers/changes.js
@@ -0,0 +1,385 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  RESET_CHANGES,
+  TRACK_CHANGE,
+} = require("resource://devtools/client/inspector/changes/actions/index.js");
+
+/**
+ * Return a deep clone of the given state object.
+ *
+ * @param {Object} state
+ * @return {Object}
+ */
+function cloneState(state = {}) {
+  return Object.entries(state).reduce((sources, [sourceId, source]) => {
+    sources[sourceId] = {
+      ...source,
+      rules: Object.entries(source.rules).reduce((rules, [ruleId, rule]) => {
+        rules[ruleId] = {
+          ...rule,
+          selectors: rule.selectors.slice(0),
+          children: rule.children.slice(0),
+          add: rule.add.slice(0),
+          remove: rule.remove.slice(0),
+        };
+
+        return rules;
+      }, {}),
+    };
+
+    return sources;
+  }, {});
+}
+
+/**
+ * Given information about a CSS rule and its ancestor rules (@media, @supports, etc),
+ * create entries in the given rules collection for each rule and assign parent/child
+ * dependencies.
+ *
+ * @param {Object} ruleData
+ *        Information about a CSS rule:
+ *        {
+ *          id:        {String}
+ *                     Unique rule id.
+ *          selectors: {Array}
+ *                     Array of CSS selector text
+ *          ancestors: {Array}
+ *                     Flattened CSS rule tree of the rule's ancestors with the root rule
+ *                     at the beginning of the array and the leaf rule at the end.
+ *          ruleIndex: {Array}
+ *                     Indexes of each ancestor rule within its parent rule.
+ *        }
+ *
+ * @param {Object} rules
+ *        Collection of rules to be mutated.
+ *        This is a reference to the corresponding `rules` object from the state.
+ *
+ * @return {Object}
+ *         Entry for the CSS rule created the given collection of rules.
+ */
+function createRule(ruleData, rules) {
+  // Append the rule data to the flattened CSS rule tree with its ancestors.
+  const ruleAncestry = [...ruleData.ancestors, { ...ruleData }];
+
+  return (
+    ruleAncestry
+      .map((rule, index) => {
+        // Ensure each rule has ancestors excluding itself (expand the flattened rule tree).
+        rule.ancestors = ruleAncestry.slice(0, index);
+        // Ensure each rule has a selector text.
+        // For the purpose of displaying in the UI, we treat at-rules as selectors.
+        if (!rule.selectors || !rule.selectors.length) {
+          // Display the @type label if there's one
+          let selector = rule.typeName ? rule.typeName + " " : "";
+          selector +=
+            rule.conditionText ||
+            rule.name ||
+            rule.keyText ||
+            rule.selectorText;
+
+          rule.selectors = [selector];
+        }
+
+        return rule.id;
+      })
+      // Then, create new entries in the rules collection and assign dependencies.
+      .map((ruleId, index, array) => {
+        const { selectors } = ruleAncestry[index];
+        const prevRuleId = array[index - 1];
+        const nextRuleId = array[index + 1];
+
+        // Create an entry for this ruleId if one does not exist.
+        if (!rules[ruleId]) {
+          rules[ruleId] = {
+            ruleId,
+            isNew: false,
+            selectors,
+            add: [],
+            remove: [],
+            children: [],
+            parent: null,
+          };
+        }
+
+        // The next ruleId is lower in the rule tree, therefore it's a child of this rule.
+        if (nextRuleId && !rules[ruleId].children.includes(nextRuleId)) {
+          rules[ruleId].children.push(nextRuleId);
+        }
+
+        // The previous ruleId is higher in the rule tree, therefore it's the parent.
+        if (prevRuleId) {
+          rules[ruleId].parent = prevRuleId;
+        }
+
+        return rules[ruleId];
+      })
+      // Finally, return the last rule in the array which is the rule we set out to create.
+      .pop()
+  );
+}
+
+function removeRule(ruleId, rules) {
+  const rule = rules[ruleId];
+
+  // First, remove this rule's id from its parent's list of children
+  if (rule.parent && rules[rule.parent]) {
+    rules[rule.parent].children = rules[rule.parent].children.filter(
+      childRuleId => {
+        return childRuleId !== ruleId;
+      }
+    );
+
+    // Remove the parent rule if it has no children left.
+    if (!rules[rule.parent].children.length) {
+      removeRule(rule.parent, rules);
+    }
+  }
+
+  delete rules[ruleId];
+}
+
+/**
+ * Aggregated changes grouped by sources (stylesheet/element), which contain rules,
+ * which contain collections of added and removed CSS declarations.
+ *
+ * Structure:
+ *    <sourceId>: {
+ *      type: // {String} One of: "stylesheet", "inline" or "element"
+ *      href: // {String|null} Stylesheet or document URL; null for inline stylesheets
+ *      rules: {
+ *        <ruleId>: {
+ *          ruleId:      // {String} <ruleId> of this rule
+ *          isNew:       // {Boolean} Whether the tracked rule was created at runtime,
+ *                       //           meaning it didn't originally exist in the source.
+ *          selectors:   // {Array} of CSS selectors or CSS at-rule text.
+ *                       //         The array has just one item if the selector is never
+ *                       //         changed. When the rule's selector is changed, the new
+ *                       //         selector is pushed onto this array.
+ *          children: [] // {Array} of <ruleId> for child rules of this rule
+ *          parent:      // {String} <ruleId> of the parent rule
+ *          add: [       // {Array} of objects with CSS declarations
+ *            {
+ *              property:    // {String} CSS property name
+ *              value:       // {String} CSS property value
+ *              index:       // {Number} Position of the declaration within its CSS rule
+ *            }
+ *            ... // more declarations
+ *          ],
+ *          remove: []   // {Array} of objects with CSS declarations
+ *        }
+ *        ... // more rules
+ *      }
+ *    }
+ *    ... // more sources
+ */
+const INITIAL_STATE = {};
+
+const reducers = {
+  /**
+   * CSS changes are collected on the server by the ChangesActor which dispatches them to
+   * the client as atomic operations: a rule/declaration updated, added or removed.
+   *
+   * By design, the ChangesActor has no big-picture context of all the collected changes.
+   * It only holds the stack of atomic changes. This makes it roboust for many use cases:
+   * building a diff-view, supporting undo/redo, offline persistence, etc. Consumers,
+   * like the Changes panel, get to massage the data for their particular purposes.
+   *
+   * Here in the reducer, we aggregate incoming changes to build a human-readable diff
+   * shown in the Changes panel.
+   * - added / removed declarations are grouped by CSS rule. Rules are grouped by their
+   *   parent rules (@media, @supports, @keyframes, etc.); Rules belong to sources
+   *   (stylesheets, inline styles)
+   * - declarations have an index corresponding to their position in the CSS rule. This
+   *   allows tracking of multiple declarations with the same property name.
+   * - repeated changes a declaration will show only the original removal and the latest
+   *   addition;
+   * - when a declaration is removed, we update the indices of other tracked declarations
+   *   in the same rule which may have changed position in the rule as a result;
+   * - changes which cancel each other out (i.e. return to original) are both removed
+   *   from the store;
+   * - when changes cancel each other out leaving the rule unchanged, the rule is removed
+   *   from the store. Its parent rule is removed as well if it too ends up unchanged.
+   */
+  // eslint-disable-next-line complexity
+  [TRACK_CHANGE](state, { change }) {
+    const defaults = {
+      selector: null,
+      source: {},
+      ancestors: [],
+      add: [],
+      remove: [],
+    };
+
+    change = { ...defaults, ...change };
+    state = cloneState(state);
+
+    const { selector, ancestors, ruleIndex } = change;
+    const sourceId = change.source.id;
+    const ruleId = change.id;
+
+    // Copy or create object identifying the source (styelsheet/element) for this change.
+    const source = Object.assign({}, state[sourceId], change.source);
+    // Copy or create collection of all rules ever changed in this source.
+    const rules = Object.assign({}, source.rules);
+    // Reference or create object identifying the rule for this change.
+    const rule = rules[ruleId]
+      ? rules[ruleId]
+      : createRule(
+          { id: change.id, selectors: [selector], ancestors, ruleIndex },
+          rules
+        );
+
+    // Mark the rule if it was created at runtime as a result of an "Add Rule" action.
+    if (change.type === "rule-add") {
+      rule.isNew = true;
+    }
+
+    // If the first selector tracked for this rule is identical to the incoming selector,
+    // reduce the selectors array to a single one. This handles the case for renaming a
+    // selector back to its original name. It has no side effects for other changes which
+    // preserve the selector.
+    // If the rule was created at runtime, always reduce the selectors array to one item.
+    // Changes to the new rule's selector always overwrite the original selector.
+    // If the selectors are different, push the incoming one to the end of the array to
+    // signify that the rule has changed selector. The last item is the current selector.
+    if (rule.selectors[0] === selector || rule.isNew) {
+      rule.selectors = [selector];
+    } else {
+      rule.selectors.push(selector);
+    }
+
+    if (change.remove?.length) {
+      for (const decl of change.remove) {
+        // Find the position of any added declaration which matches the incoming
+        // declaration to be removed.
+        const addIndex = rule.add.findIndex(addDecl => {
+          return (
+            addDecl.index === decl.index &&
+            addDecl.property === decl.property &&
+            addDecl.value === decl.value
+          );
+        });
+
+        // Find the position of any removed declaration which matches the incoming
+        // declaration to be removed. It's possible to get duplicate remove operations
+        // when, for example, disabling a declaration then deleting it.
+        const removeIndex = rule.remove.findIndex(removeDecl => {
+          return (
+            removeDecl.index === decl.index &&
+            removeDecl.property === decl.property &&
+            removeDecl.value === decl.value
+          );
+        });
+
+        // Track the remove operation only if the property was not previously introduced
+        // by an add operation. This ensures repeated changes of the same property
+        // register as a single remove operation of its original value. Avoid tracking the
+        // remove declaration if already tracked (happens on disable followed by delete).
+        if (addIndex < 0 && removeIndex < 0) {
+          rule.remove.push(decl);
+        }
+
+        // Delete any previous add operation which would be canceled out by this remove.
+        if (rule.add[addIndex]) {
+          rule.add.splice(addIndex, 1);
+        }
+
+        // Update the indexes of previously tracked declarations which follow this removed
+        // one so future tracking continues to point to the right declarations.
+        if (change.type === "declaration-remove") {
+          rule.add = rule.add.map(addDecl => {
+            if (addDecl.index > decl.index) {
+              addDecl.index--;
+            }
+
+            return addDecl;
+          });
+
+          rule.remove = rule.remove.map(removeDecl => {
+            if (removeDecl.index > decl.index) {
+              removeDecl.index--;
+            }
+
+            return removeDecl;
+          });
+        }
+      }
+    }
+
+    if (change.add?.length) {
+      for (const decl of change.add) {
+        // Find the position of any removed declaration which matches the incoming
+        // declaration to be added.
+        const removeIndex = rule.remove.findIndex(removeDecl => {
+          return (
+            removeDecl.index === decl.index &&
+            removeDecl.value === decl.value &&
+            removeDecl.property === decl.property
+          );
+        });
+
+        // Find the position of any added declaration which matches the incoming
+        // declaration to be added in case we need to replace it.
+        const addIndex = rule.add.findIndex(addDecl => {
+          return (
+            addDecl.index === decl.index && addDecl.property === decl.property
+          );
+        });
+
+        if (rule.remove[removeIndex]) {
+          // Delete any previous remove operation which would be canceled out by this add.
+          rule.remove.splice(removeIndex, 1);
+        } else if (rule.add[addIndex]) {
+          // Replace previous add operation for declaration at this index.
+          rule.add.splice(addIndex, 1, decl);
+        } else {
+          // Track new add operation.
+          rule.add.push(decl);
+        }
+      }
+    }
+
+    // Remove the rule if none of its declarations or selector have changed,
+    // but skip cleanup if the selector is in process of being renamed (there are two
+    // changes happening in quick succession: selector-remove + selector-add) or if the
+    // rule was created at runtime (allow empty new rules to persist).
+    if (
+      !rule.add.length &&
+      !rule.remove.length &&
+      rule.selectors.length === 1 &&
+      !change.type.startsWith("selector-") &&
+      !rule.isNew
+    ) {
+      removeRule(ruleId, rules);
+      source.rules = { ...rules };
+    } else {
+      source.rules = { ...rules, [ruleId]: rule };
+    }
+
+    // Remove information about the source if none of its rules changed.
+    if (!Object.keys(source.rules).length) {
+      delete state[sourceId];
+    } else {
+      state[sourceId] = source;
+    }
+
+    return state;
+  },
+
+  [RESET_CHANGES](state) {
+    return INITIAL_STATE;
+  },
+};
+
+module.exports = function (state = INITIAL_STATE, action) {
+  const reducer = reducers[action.type];
+  if (!reducer) {
+    return state;
+  }
+  return reducer(state, action);
+};
diff --git a/devtools/client/inspector/changes/reducers/moz.build b/devtools/client/inspector/changes/reducers/moz.build
new file mode 100644
index 0000000000..f3ea9a1bfc
--- /dev/null
+++ b/devtools/client/inspector/changes/reducers/moz.build
@@ -0,0 +1,9 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+DevToolsModules(
+    "changes.js",
+)
diff --git a/devtools/client/inspector/changes/selectors/changes.js b/devtools/client/inspector/changes/selectors/changes.js
new file mode 100644
index 0000000000..a6b99e4579
--- /dev/null
+++ b/devtools/client/inspector/changes/selectors/changes.js
@@ -0,0 +1,261 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+loader.lazyRequireGetter(
+  this,
+  "getTabPrefs",
+  "resource://devtools/shared/indentation.js",
+  true
+);
+
+const {
+  getSourceForDisplay,
+} = require("resource://devtools/client/inspector/changes/utils/changes-utils.js");
+
+/**
+ * In the Redux state, changed CSS rules are grouped by source (stylesheet) and stored in
+ * a single level array, regardless of nesting.
+ * This method returns a nested tree structure of the changed CSS rules so the React
+ * consumer components can traverse it easier when rendering the nested CSS rules view.
+ * Keeping this interface updated allows the Redux state structure to change without
+ * affecting the consumer components.
+ *
+ * @param {Object} state
+ *        Redux slice for tracked changes.
+ * @param {Object} filter
+ *        Object with optional filters to use. Has the following properties:
+ *        - sourceIds: {Array}
+ *          Use only subtrees of sources matching source ids from this array.
+ *        - ruleIds: {Array}
+ *          Use only rules matching rule ids from this array. If the array includes ids
+ *          of ancestor rules (@media, @supports), their nested rules will be included.
+ * @return {Object}
+ */
+function getChangesTree(state, filter = {}) {
+  // Use or assign defaults of sourceId and ruleId arrays by which to filter the tree.
+  const { sourceIds: sourceIdsFilter = [], ruleIds: rulesIdsFilter = [] } =
+    filter;
+  /**
+   * Recursively replace a rule's array of child rule ids with the referenced child rules.
+   * Mark visited rules so as not to handle them (and their children) again.
+   *
+   * Returns the rule object with expanded children or null if previously visited.
+   *
+   * @param  {String} ruleId
+   * @param  {Object} rule
+   * @param  {Array} rules
+   * @param  {Set} visitedRules
+   * @return {Object|null}
+   */
+  function expandRuleChildren(ruleId, rule, rules, visitedRules) {
+    if (visitedRules.has(ruleId)) {
+      return null;
+    }
+
+    visitedRules.add(ruleId);
+
+    return {
+      ...rule,
+      children: rule.children.map(childRuleId =>
+        expandRuleChildren(childRuleId, rules[childRuleId], rules, visitedRules)
+      ),
+    };
+  }
+
+  return Object.entries(state)
+    .filter(([sourceId, source]) => {
+      // Use only matching sources if an array to filter by was provided.
+      if (sourceIdsFilter.length) {
+        return sourceIdsFilter.includes(sourceId);
+      }
+
+      return true;
+    })
+    .reduce((sourcesObj, [sourceId, source]) => {
+      const { rules } = source;
+      // Log of visited rules in this source. Helps avoid duplication when traversing the
+      // descendant rule tree. This Set is unique per source. It will be passed down to
+      // be populated with ids of rules once visited. This ensures that only visited rules
+      // unique to this source will be skipped and prevents skipping identical rules from
+      // other sources (ex: rules with the same selector and the same index).
+      const visitedRules = new Set();
+
+      // Build a new collection of sources keyed by source id.
+      sourcesObj[sourceId] = {
+        ...source,
+        // Build a new collection of rules keyed by rule id.
+        rules: Object.entries(rules)
+          .filter(([ruleId, rule]) => {
+            // Use only matching rules if an array to filter by was provided.
+            if (rulesIdsFilter.length) {
+              return rulesIdsFilter.includes(ruleId);
+            }
+
+            return true;
+          })
+          .reduce((rulesObj, [ruleId, rule]) => {
+            // Expand the rule's array of child rule ids with the referenced child rules.
+            // Skip exposing null values which mean the rule was previously visited
+            // as part of an ancestor descendant tree.
+            const expandedRule = expandRuleChildren(
+              ruleId,
+              rule,
+              rules,
+              visitedRules
+            );
+            if (expandedRule !== null) {
+              rulesObj[ruleId] = expandedRule;
+            }
+
+            return rulesObj;
+          }, {}),
+      };
+
+      return sourcesObj;
+    }, {});
+}
+
+/**
+ * Build the CSS text of a stylesheet with the changes aggregated in the Redux state.
+ * If filters for rule id or source id are provided, restrict the changes to the matching
+ * sources and rules.
+ *
+ * Code comments with the source origin are put above of the CSS rule (or group of
+ * rules). Removed CSS declarations are written commented out. Added CSS declarations are
+ * written as-is.
+ *
+ * @param  {Object} state
+ *         Redux slice for tracked changes.
+ * @param  {Object} filter
+ *         Object with optional source and rule filters. See getChangesTree()
+ * @return {String}
+ *         CSS stylesheet text.
+ */
+
+// For stylesheet sources, the stylesheet filename and full path are used:
+//
+// /* styles.css | https://example.com/styles.css */
+//
+// .selector {
+//  /* property: oldvalue; */
+//  property: value;
+// }
+
+// For inline stylesheet sources, the stylesheet index and host document URL are used:
+//
+// /* Inline #1 | https://example.com */
+//
+// .selector {
+//  /* property: oldvalue; */
+//  property: value;
+// }
+
+// For element style attribute sources, the unique selector generated for the element
+// and the host document URL are used:
+//
+// /* Element (div) | https://example.com */
+//
+// div:nth-child(1) {
+//  /* property: oldvalue; */
+//  property: value;
+// }
+function getChangesStylesheet(state, filter) {
+  const changeTree = getChangesTree(state, filter);
+  // Get user prefs about indentation style.
+  const { indentUnit, indentWithTabs } = getTabPrefs();
+  const indentChar = indentWithTabs
+    ? "\t".repeat(indentUnit)
+    : " ".repeat(indentUnit);
+
+  /**
+   * If the rule has just one item in its array of selector versions, return it as-is.
+   * If it has more than one, build a string using the first selector commented-out
+   * and the last selector as-is. This indicates that a rule's selector has changed.
+   *
+   * @param  {Array} selectors
+   *         History of selector versions if changed over time.
+   *         Array with a single item (the original selector) if never changed.
+   * @param  {Number} level
+   *         Level of nesting within a CSS rule tree.
+   * @return {String}
+   */
+  function writeSelector(selectors = [], level) {
+    const indent = indentChar.repeat(level);
+    let selectorText;
+    switch (selectors.length) {
+      case 0:
+        selectorText = "";
+        break;
+      case 1:
+        selectorText = `${indent}${selectors[0]}`;
+        break;
+      default:
+        selectorText =
+          `${indent}/* ${selectors[0]} { */\n` +
+          `${indent}${selectors[selectors.length - 1]}`;
+    }
+
+    return selectorText;
+  }
+
+  function writeRule(ruleId, rule, level) {
+    // Write nested rules, if any.
+    let ruleBody = rule.children.reduce((str, childRule) => {
+      str += writeRule(childRule.ruleId, childRule, level + 1);
+      return str;
+    }, "");
+
+    // Write changed CSS declarations.
+    ruleBody += writeDeclarations(rule.remove, rule.add, level + 1);
+
+    const indent = indentChar.repeat(level);
+    const selectorText = writeSelector(rule.selectors, level);
+    return `\n${selectorText} {${ruleBody}\n${indent}}`;
+  }
+
+  function writeDeclarations(remove = [], add = [], level) {
+    const indent = indentChar.repeat(level);
+    const removals = remove
+      // Sort declarations in the order in which they exist in the original CSS rule.
+      .sort((a, b) => a.index > b.index)
+      .reduce((str, { property, value }) => {
+        str += `\n${indent}/* ${property}: ${value}; */`;
+        return str;
+      }, "");
+
+    const additions = add
+      // Sort declarations in the order in which they exist in the original CSS rule.
+      .sort((a, b) => a.index > b.index)
+      .reduce((str, { property, value }) => {
+        str += `\n${indent}${property}: ${value};`;
+        return str;
+      }, "");
+
+    return removals + additions;
+  }
+
+  // Iterate through all sources in the change tree and build a CSS stylesheet string.
+  return Object.entries(changeTree).reduce(
+    (stylesheetText, [sourceId, source]) => {
+      const { href, rules } = source;
+      // Write code comment with source origin
+      stylesheetText += `\n/* ${getSourceForDisplay(source)} | ${href} */\n`;
+      // Write CSS rules
+      stylesheetText += Object.entries(rules).reduce((str, [ruleId, rule]) => {
+        // Add a new like only after top-level rules (level == 0)
+        str += writeRule(ruleId, rule, 0) + "\n";
+        return str;
+      }, "");
+
+      return stylesheetText;
+    },
+    ""
+  );
+}
+
+module.exports = {
+  getChangesTree,
+  getChangesStylesheet,
+};
diff --git a/devtools/client/inspector/changes/selectors/moz.build b/devtools/client/inspector/changes/selectors/moz.build
new file mode 100644
index 0000000000..f3ea9a1bfc
--- /dev/null
+++ b/devtools/client/inspector/changes/selectors/moz.build
@@ -0,0 +1,9 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+DevToolsModules(
+    "changes.js",
+)
diff --git a/devtools/client/inspector/changes/test/browser.toml b/devtools/client/inspector/changes/test/browser.toml
new file mode 100644
index 0000000000..407a960e92
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser.toml
@@ -0,0 +1,45 @@
+[DEFAULT]
+tags = "devtools"
+subsuite = "devtools"
+support-files = [
+  "head.js",
+  "!/devtools/client/inspector/test/head.js",
+  "!/devtools/client/inspector/test/shared-head.js",
+  "!/devtools/client/inspector/rules/test/head.js",
+  "!/devtools/client/shared/test/shared-head.js",
+  "!/devtools/client/shared/test/telemetry-test-helpers.js",
+  "!/devtools/client/shared/test/highlighter-test-actor.js",
+]
+
+["browser_changes_background_tracking.js"]
+
+["browser_changes_copy_all_changes.js"]
+
+["browser_changes_copy_declaration.js"]
+
+["browser_changes_copy_rule.js"]
+
+["browser_changes_declaration_add_special_character.js"]
+
+["browser_changes_declaration_disable.js"]
+
+["browser_changes_declaration_duplicate.js"]
+
+["browser_changes_declaration_edit_value.js"]
+
+["browser_changes_declaration_identical_rules.js"]
+skip-if = ["a11y_checks"] # Bugs 1858041 and 1849028 to investigate intermittent a11y_checks results (fails on Autoland, passes on Try)
+
+["browser_changes_declaration_remove.js"]
+
+["browser_changes_declaration_remove_ahead.js"]
+
+["browser_changes_declaration_remove_disabled.js"]
+
+["browser_changes_declaration_rename.js"]
+
+["browser_changes_nested_rules.js"]
+
+["browser_changes_rule_add.js"]
+
+["browser_changes_rule_selector.js"]
diff --git a/devtools/client/inspector/changes/test/browser_changes_background_tracking.js b/devtools/client/inspector/changes/test/browser_changes_background_tracking.js
new file mode 100644
index 0000000000..8ebac2a23a
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_background_tracking.js
@@ -0,0 +1,46 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that CSS changes are collected in the background without the Changes panel visible
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+      color: red;
+    }
+  </style>
+  <div></div>
+`;
+
+add_task(async function () {
+  info("Ensure Changes panel is NOT the default panel; use Computed panel");
+  await pushPref("devtools.inspector.activeSidebar", "computedview");
+
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+
+  await selectNode("div", inspector);
+  const prop = getTextProperty(ruleView, 1, { color: "red" });
+
+  info("Disable the first CSS declaration");
+  await togglePropStatus(ruleView, prop);
+
+  info("Select the Changes panel");
+  const { document: doc, store } = selectChangesView(inspector);
+  const onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  const onResetChanges = waitForDispatch(store, "RESET_CHANGES");
+
+  info("Wait for change to be tracked");
+  await onTrackChange;
+  const removedDeclarations = getRemovedDeclarations(doc);
+  is(removedDeclarations.length, 1, "One declaration was tracked as removed");
+
+  // Test for Bug 1656477. Check that changes are not cleared immediately afterwards.
+  info("Wait to see if the RESET_CHANGES action is dispatched unexpecteldy");
+  const sym = Symbol();
+  const onTimeout = wait(500).then(() => sym);
+  const raceResult = await Promise.any([onResetChanges, onTimeout]);
+  Assert.strictEqual(raceResult, sym, "RESET_CHANGES has not been dispatched");
+});
diff --git a/devtools/client/inspector/changes/test/browser_changes_copy_all_changes.js b/devtools/client/inspector/changes/test/browser_changes_copy_all_changes.js
new file mode 100644
index 0000000000..119fe22585
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_copy_all_changes.js
@@ -0,0 +1,53 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that the Changes panel Copy All Changes button will populate the
+// clipboard with a summary of just the changed declarations.
+
+const TEST_URI = `
+  <style type='text/css'>
+  div {
+    color: red;
+    margin: 0;
+  }
+  </style>
+  <div></div>
+`;
+
+// Indentation is important. A strict check will be done against the clipboard content.
+const EXPECTED_CLIPBOARD = `
+/* Inline #0 | data:text/html;charset=utf-8,${TEST_URI} */
+
+div {
+  /* color: red; */
+  color: green;
+}
+`;
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const changesView = selectChangesView(inspector);
+  const { document: panelDoc, store } = changesView;
+
+  await selectNode("div", inspector);
+  const onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  await updateDeclaration(ruleView, 1, { color: "red" }, { color: "green" });
+  await onTrackChange;
+
+  info(
+    "Check that clicking the Copy All Changes button copies all changes to the clipboard."
+  );
+  const button = panelDoc.querySelector(".changes__copy-all-changes-button");
+  await waitForClipboardPromise(
+    () => button.click(),
+    () => checkClipboardData(EXPECTED_CLIPBOARD)
+  );
+});
+
+function checkClipboardData(expected) {
+  const actual = SpecialPowers.getClipboardData("text/plain");
+  return decodeURIComponent(actual).trim() === expected.trim();
+}
diff --git a/devtools/client/inspector/changes/test/browser_changes_copy_declaration.js b/devtools/client/inspector/changes/test/browser_changes_copy_declaration.js
new file mode 100644
index 0000000000..0f2c7f68e6
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_copy_declaration.js
@@ -0,0 +1,67 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that the Changes panel Copy Declaration context menu item will populate the
+// clipboard with the changed declaration.
+
+const TEST_URI = `
+  <style type='text/css'>
+  div {
+    color: red;
+    margin: 0;
+  }
+  </style>
+  <div></div>
+`;
+
+const EXPECTED_CLIPBOARD_REMOVED = `/* color: red; */`;
+const EXPECTED_CLIPBOARD_ADDED = `color: green;`;
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const changesView = selectChangesView(inspector);
+  const { document: panelDoc, store } = changesView;
+
+  await selectNode("div", inspector);
+  const onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  await updateDeclaration(ruleView, 1, { color: "red" }, { color: "green" });
+  await onTrackChange;
+
+  info(
+    "Click the Copy Declaration context menu item for the removed declaration"
+  );
+  const removeDecl = getRemovedDeclarations(panelDoc);
+  const addDecl = getAddedDeclarations(panelDoc);
+
+  let menu = await getChangesContextMenu(changesView, removeDecl[0].element);
+  let menuItem = menu.items.find(
+    item => item.id === "changes-contextmenu-copy-declaration"
+  );
+  await waitForClipboardPromise(
+    () => menuItem.click(),
+    () => checkClipboardData(EXPECTED_CLIPBOARD_REMOVED)
+  );
+
+  info("Hiding menu");
+  menu.hide(document);
+
+  info(
+    "Click the Copy Declaration context menu item for the added declaration"
+  );
+  menu = await getChangesContextMenu(changesView, addDecl[0].element);
+  menuItem = menu.items.find(
+    item => item.id === "changes-contextmenu-copy-declaration"
+  );
+  await waitForClipboardPromise(
+    () => menuItem.click(),
+    () => checkClipboardData(EXPECTED_CLIPBOARD_ADDED)
+  );
+});
+
+function checkClipboardData(expected) {
+  const actual = SpecialPowers.getClipboardData("text/plain");
+  return actual.trim() === expected.trim();
+}
diff --git a/devtools/client/inspector/changes/test/browser_changes_copy_rule.js b/devtools/client/inspector/changes/test/browser_changes_copy_rule.js
new file mode 100644
index 0000000000..4c2a347e8e
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_copy_rule.js
@@ -0,0 +1,64 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that the Changes panel Copy Rule button and context menu will populate the
+// clipboard with the entire contents of the changed rule, including unchanged properties.
+
+const TEST_URI = `
+  <style type='text/css'>
+  div {
+    color: red;
+    margin: 0;
+  }
+  </style>
+  <div></div>
+`;
+
+// Indentation is important. A strict check will be done against the clipboard content.
+const EXPECTED_CLIPBOARD = `
+  div {
+    color: green;
+    margin: 0;
+  }
+`;
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const changesView = selectChangesView(inspector);
+  const { document: panelDoc, store } = changesView;
+
+  await selectNode("div", inspector);
+  const onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  await updateDeclaration(ruleView, 1, { color: "red" }, { color: "green" });
+  await onTrackChange;
+
+  info("Click the Copy Rule button and expect the changed rule on clipboard");
+  const button = panelDoc.querySelector(".changes__copy-rule-button");
+  await waitForClipboardPromise(
+    () => button.click(),
+    () => checkClipboardData(EXPECTED_CLIPBOARD)
+  );
+
+  emptyClipboard();
+
+  info(
+    "Click the Copy Rule context menu item and expect the changed rule on the clipboard"
+  );
+  const addDecl = getAddedDeclarations(panelDoc);
+  const menu = await getChangesContextMenu(changesView, addDecl[0].element);
+  const menuItem = menu.items.find(
+    item => item.id === "changes-contextmenu-copy-rule"
+  );
+  await waitForClipboardPromise(
+    () => menuItem.click(),
+    () => checkClipboardData(EXPECTED_CLIPBOARD)
+  );
+});
+
+function checkClipboardData(expected) {
+  const actual = SpecialPowers.getClipboardData("text/plain");
+  return actual.trim() === expected.trim();
+}
diff --git a/devtools/client/inspector/changes/test/browser_changes_declaration_add_special_character.js b/devtools/client/inspector/changes/test/browser_changes_declaration_add_special_character.js
new file mode 100644
index 0000000000..65b0092b9c
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_declaration_add_special_character.js
@@ -0,0 +1,78 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that adding new CSS properties with special characters in the property
+// name does note create duplicate entries.
+
+const PROPERTY_NAME = '"abc"';
+const INITIAL_VALUE = "foo";
+// For assertions the quotes in the property will be escaped.
+const EXPECTED_PROPERTY_NAME = '\\"abc\\"';
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+      color: red;
+    }
+  </style>
+  <div>test</div>
+`;
+
+add_task(async function addWithSpecialCharacter() {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const { document: doc, store } = selectChangesView(inspector);
+
+  await selectNode("div", inspector);
+
+  const ruleEditor = getRuleViewRuleEditor(ruleView, 1);
+  const editor = await focusEditableField(ruleView, ruleEditor.closeBrace);
+
+  const input = editor.input;
+  input.value = `${PROPERTY_NAME}: ${INITIAL_VALUE};`;
+
+  let onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Pressing return to commit and focus the new value field");
+  const onModifications = ruleView.once("ruleview-changed");
+  EventUtils.synthesizeKey("VK_RETURN", {}, ruleView.styleWindow);
+  await onModifications;
+  await onTrackChange;
+  await assertAddedDeclaration(doc, EXPECTED_PROPERTY_NAME, INITIAL_VALUE);
+
+  let newValue = "def";
+  info(`Change the CSS declaration value to ${newValue}`);
+  const prop = getTextProperty(ruleView, 1, { [PROPERTY_NAME]: INITIAL_VALUE });
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  // flushCount needs to be set to 2 once when quotes are involved.
+  await setProperty(ruleView, prop, newValue, { flushCount: 2 });
+  await onTrackChange;
+  await assertAddedDeclaration(doc, EXPECTED_PROPERTY_NAME, newValue);
+
+  newValue = "123";
+  info(`Change the CSS declaration value to ${newValue}`);
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  // 2 preview requests to flush: one for the new value and one for the
+  // autocomplete popup suggestion (even if no suggestion is displayed here).
+  await setProperty(ruleView, prop, newValue, { flushCount: 2 });
+  await onTrackChange;
+  await assertAddedDeclaration(doc, EXPECTED_PROPERTY_NAME, newValue);
+});
+
+/**
+ * Check that we only received a single added declaration with the expected
+ * value.
+ */
+async function assertAddedDeclaration(doc, expectedName, expectedValue) {
+  await waitFor(() => {
+    const addDecl = getAddedDeclarations(doc);
+    return (
+      addDecl.length == 1 &&
+      addDecl[0].value == expectedValue &&
+      addDecl[0].property == expectedName
+    );
+  }, "Got the expected declaration");
+  is(getAddedDeclarations(doc).length, 1, "Only one added declaration");
+  is(getRemovedDeclarations(doc).length, 0, "No removed declaration");
+}
diff --git a/devtools/client/inspector/changes/test/browser_changes_declaration_disable.js b/devtools/client/inspector/changes/test/browser_changes_declaration_disable.js
new file mode 100644
index 0000000000..4c7141cdc6
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_declaration_disable.js
@@ -0,0 +1,48 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that toggling a CSS declaration in the Rule view is tracked.
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+      color: red;
+    }
+  </style>
+  <div></div>
+`;
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const { document: doc, store } = selectChangesView(inspector);
+
+  await selectNode("div", inspector);
+  const prop = getTextProperty(ruleView, 1, { color: "red" });
+
+  let onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Disable the first declaration");
+  await togglePropStatus(ruleView, prop);
+  info("Wait for change to be tracked");
+  await onTrackChange;
+
+  let removedDeclarations = getRemovedDeclarations(doc);
+  is(
+    removedDeclarations.length,
+    1,
+    "Only one declaration was tracked as removed"
+  );
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Re-enable the first declaration");
+  await togglePropStatus(ruleView, prop);
+  info("Wait for change to be tracked");
+  await onTrackChange;
+
+  const addedDeclarations = getAddedDeclarations(doc);
+  removedDeclarations = getRemovedDeclarations(doc);
+  is(addedDeclarations.length, 0, "No declarations were tracked as added");
+  is(removedDeclarations.length, 0, "No declarations were tracked as removed");
+});
diff --git a/devtools/client/inspector/changes/test/browser_changes_declaration_duplicate.js b/devtools/client/inspector/changes/test/browser_changes_declaration_duplicate.js
new file mode 100644
index 0000000000..1d3423992e
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_declaration_duplicate.js
@@ -0,0 +1,107 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that adding duplicate declarations to the Rule view is shown in the Changes panel.
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+    }
+  </style>
+  <div></div>
+`;
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const { document: doc, store } = selectChangesView(inspector);
+
+  await selectNode("div", inspector);
+  await testAddDuplicateDeclarations(ruleView, store, doc);
+  await testChangeDuplicateDeclarations(ruleView, store, doc);
+  await testRemoveDuplicateDeclarations(ruleView, store, doc);
+});
+
+async function testAddDuplicateDeclarations(ruleView, store, doc) {
+  info(`Test that adding declarations with the same property name and value
+        are both tracked.`);
+
+  let onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Add CSS declaration");
+  await addProperty(ruleView, 1, "color", "red");
+  info("Wait for the change to be tracked");
+  await onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Add duplicate CSS declaration");
+  await addProperty(ruleView, 1, "color", "red");
+  info("Wait for the change to be tracked");
+  await onTrackChange;
+
+  await waitFor(() => {
+    const decls = getAddedDeclarations(doc);
+    return decls.length == 2 && decls[1].value == "red";
+  }, "Two declarations were tracked as added");
+  const addDecl = getAddedDeclarations(doc);
+  is(addDecl[0].value, "red", "First declaration has correct property value");
+  is(
+    addDecl[0].value,
+    addDecl[1].value,
+    "First and second declarations have identical property values"
+  );
+}
+
+async function testChangeDuplicateDeclarations(ruleView, store, doc) {
+  info(
+    "Test that changing one of the duplicate declarations won't change the other"
+  );
+  const prop = getTextProperty(ruleView, 1, { color: "red" });
+
+  info("Change the value of the first of the duplicate declarations");
+  const onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  await setProperty(ruleView, prop, "black");
+  info("Wait for the change to be tracked");
+  await onTrackChange;
+
+  await waitFor(
+    () => getAddedDeclarations(doc).length == 2,
+    "Two declarations were tracked as added"
+  );
+  const addDecl = getAddedDeclarations(doc);
+  is(addDecl[0].value, "black", "First declaration has changed property value");
+  is(
+    addDecl[1].value,
+    "red",
+    "Second declaration has not changed property value"
+  );
+}
+
+async function testRemoveDuplicateDeclarations(ruleView, store, doc) {
+  info(`Test that removing the first of the duplicate declarations
+        will not remove the second.`);
+
+  const prop = getTextProperty(ruleView, 1, { color: "black" });
+
+  info("Remove first declaration");
+  const onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  await removeProperty(ruleView, prop);
+  info("Wait for the change to be tracked");
+  await onTrackChange;
+
+  await waitFor(
+    () => getAddedDeclarations(doc).length == 1,
+    "One declaration was tracked as added"
+  );
+  const addDecl = getAddedDeclarations(doc);
+  const removeDecl = getRemovedDeclarations(doc);
+  // Expect no remove operation tracked because it cancels out the original add operation.
+  is(removeDecl.length, 0, "No declaration was tracked as removed");
+  is(addDecl.length, 1, "Just one declaration left tracked as added");
+  is(
+    addDecl[0].value,
+    "red",
+    "Leftover declaration has property value of the former second declaration"
+  );
+}
diff --git a/devtools/client/inspector/changes/test/browser_changes_declaration_edit_value.js b/devtools/client/inspector/changes/test/browser_changes_declaration_edit_value.js
new file mode 100644
index 0000000000..588513a274
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_declaration_edit_value.js
@@ -0,0 +1,170 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that editing the value of a CSS declaration in the Rule view is tracked.
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+      color: red;
+      font-family: "courier";
+    }
+  </style>
+  <div>test</div>
+`;
+
+/*
+  This object contains iteration steps to modify various CSS properties of the
+  test element, keyed by property name,.
+  Each value is an array which will be iterated over in order and the `value`
+  property will be used to update the value of the property.
+  The `add` and `remove` objects hold the expected values of the tracked declarations
+  shown in the Changes panel. If `add` or `remove` are null, it means we don't expect
+  any corresponding tracked declaration to show up in the Changes panel.
+ */
+const ITERATIONS = {
+  color: [
+    // No changes should be tracked if the value did not actually change.
+    {
+      value: "red",
+      add: null,
+      remove: null,
+    },
+    // Changing the priority flag "!important" should be tracked.
+    {
+      value: "red !important",
+      add: { value: "red !important" },
+      remove: { value: "red" },
+    },
+    // Repeated changes should still show the original value as the one removed.
+    {
+      value: "blue",
+      add: { value: "blue" },
+      remove: { value: "red" },
+    },
+    // Restoring the original value should clear tracked changes.
+    {
+      value: "red",
+      add: null,
+      remove: null,
+    },
+  ],
+  "font-family": [
+    // Set a value with an opening quote, missing the closing one.
+    // The closing quote should still appear in the "add" value.
+    {
+      value: '"ar',
+      add: { value: '"ar"' },
+      remove: { value: '"courier"' },
+      // For some reason we need an additional flush the first time we set a
+      // value with a quote. Since the ruleview is manually flushed when opened
+      // openRuleView, we need to pass this information all the way down to the
+      // setProperty helper.
+      needsExtraFlush: true,
+    },
+    // Add an escaped character
+    {
+      value: '"ar\\i',
+      add: { value: '"ar\\i"' },
+      remove: { value: '"courier"' },
+    },
+    // Add some more text
+    {
+      value: '"ar\\ia',
+      add: { value: '"ar\\ia"' },
+      remove: { value: '"courier"' },
+    },
+    // Remove the backslash
+    {
+      value: '"aria',
+      add: { value: '"aria"' },
+      remove: { value: '"courier"' },
+    },
+    // Add the rest of the text, still no closing quote
+    {
+      value: '"arial',
+      add: { value: '"arial"' },
+      remove: { value: '"courier"' },
+    },
+    // Restoring the original value should clear tracked changes.
+    {
+      value: '"courier"',
+      add: null,
+      remove: null,
+    },
+  ],
+};
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const { document: doc, store } = selectChangesView(inspector);
+
+  await selectNode("div", inspector);
+
+  const colorProp = getTextProperty(ruleView, 1, { color: "red" });
+  await assertEditValue(ruleView, doc, store, colorProp, ITERATIONS.color);
+
+  const fontFamilyProp = getTextProperty(ruleView, 1, {
+    "font-family": '"courier"',
+  });
+  await assertEditValue(
+    ruleView,
+    doc,
+    store,
+    fontFamilyProp,
+    ITERATIONS["font-family"]
+  );
+});
+
+async function assertEditValue(ruleView, doc, store, prop, iterations) {
+  let onTrackChange;
+  for (const { value, add, needsExtraFlush, remove } of iterations) {
+    onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+
+    info(`Change the CSS declaration value to ${value}`);
+    await setProperty(ruleView, prop, value, {
+      flushCount: needsExtraFlush ? 2 : 1,
+    });
+    info("Wait for the change to be tracked");
+    await onTrackChange;
+
+    if (add) {
+      await waitFor(() => {
+        const decl = getAddedDeclarations(doc);
+        return decl.length == 1 && decl[0].value == add.value;
+      }, "Only one declaration was tracked as added.");
+      const addDecl = getAddedDeclarations(doc);
+      is(
+        addDecl[0].value,
+        add.value,
+        `Added declaration has expected value: ${add.value}`
+      );
+    } else {
+      await waitFor(
+        () => !getAddedDeclarations(doc).length,
+        "Added declaration was cleared"
+      );
+    }
+
+    if (remove) {
+      await waitFor(
+        () => getRemovedDeclarations(doc).length == 1,
+        "Only one declaration was tracked as removed."
+      );
+      const removeDecl = getRemovedDeclarations(doc);
+      is(
+        removeDecl[0].value,
+        remove.value,
+        `Removed declaration has expected value: ${remove.value}`
+      );
+    } else {
+      await waitFor(
+        () => !getRemovedDeclarations(doc).length,
+        "Removed declaration was cleared"
+      );
+    }
+  }
+}
diff --git a/devtools/client/inspector/changes/test/browser_changes_declaration_identical_rules.js b/devtools/client/inspector/changes/test/browser_changes_declaration_identical_rules.js
new file mode 100644
index 0000000000..08ac6d173d
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_declaration_identical_rules.js
@@ -0,0 +1,71 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test tracking changes to CSS declarations in different stylesheets but in rules
+// with identical selectors.
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+      color: red;
+    }
+  </style>
+  <style type='text/css'>
+    div {
+      font-size: 1em;
+    }
+  </style>
+  <div></div>
+`;
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const { document: doc, store } = selectChangesView(inspector);
+
+  await selectNode("div", inspector);
+  const prop1 = getTextProperty(ruleView, 1, { "font-size": "1em" });
+  const prop2 = getTextProperty(ruleView, 2, { color: "red" });
+
+  let onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Disable the declaration in the first rule");
+  await togglePropStatus(ruleView, prop1);
+  info("Wait for change to be tracked");
+  await onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Disable the declaration in the second rule");
+  await togglePropStatus(ruleView, prop2);
+  info("Wait for change to be tracked");
+  await onTrackChange;
+
+  const removeDecl = getRemovedDeclarations(doc);
+  is(removeDecl.length, 2, "Two declarations tracked as removed");
+  // The last of the two matching rules shows up first in Rule view given that the
+  // specificity is the same. This is correct. If the properties were the same, the latest
+  // declaration would overwrite the first and thus show up on top.
+  is(
+    removeDecl[0].property,
+    "font-size",
+    "Correct property name for second declaration"
+  );
+  is(
+    removeDecl[0].value,
+    "1em",
+    "Correct property value for second declaration"
+  );
+  is(
+    removeDecl[1].property,
+    "color",
+    "Correct property name for first declaration"
+  );
+  is(
+    removeDecl[1].value,
+    "red",
+    "Correct property value for first declaration"
+  );
+});
diff --git a/devtools/client/inspector/changes/test/browser_changes_declaration_remove.js b/devtools/client/inspector/changes/test/browser_changes_declaration_remove.js
new file mode 100644
index 0000000000..60b61c3196
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_declaration_remove.js
@@ -0,0 +1,43 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that removing a CSS declaration from a rule in the Rule view is tracked.
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+      color: red;
+    }
+  </style>
+  <div></div>
+`;
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const { document: doc, store } = selectChangesView(inspector);
+
+  await selectNode("div", inspector);
+  const prop = getTextProperty(ruleView, 1, { color: "red" });
+
+  const onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Remove the first declaration");
+  await removeProperty(ruleView, prop);
+  info("Wait for change to be tracked");
+  await onTrackChange;
+
+  const removeDecl = getRemovedDeclarations(doc);
+  is(removeDecl.length, 1, "One declaration was tracked as removed");
+  is(
+    removeDecl[0].property,
+    "color",
+    "Correct declaration name was tracked as removed"
+  );
+  is(
+    removeDecl[0].value,
+    "red",
+    "Correct declaration value was tracked as removed"
+  );
+});
diff --git a/devtools/client/inspector/changes/test/browser_changes_declaration_remove_ahead.js b/devtools/client/inspector/changes/test/browser_changes_declaration_remove_ahead.js
new file mode 100644
index 0000000000..b249fc8198
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_declaration_remove_ahead.js
@@ -0,0 +1,53 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that the correct declaration is identified and changed after removing a
+// declaration positioned ahead of it in the same CSS rule.
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+      color: red;
+      display: block;
+    }
+  </style>
+  <div></div>
+`;
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const { document: doc, store } = selectChangesView(inspector);
+
+  await selectNode("div", inspector);
+  const prop1 = getTextProperty(ruleView, 1, { color: "red" });
+  const prop2 = getTextProperty(ruleView, 1, { display: "block" });
+
+  let onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Change the second declaration");
+  await setProperty(ruleView, prop2, "grid");
+  await onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Remove the first declaration");
+  await removeProperty(ruleView, prop1);
+  await onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Change the second declaration again");
+  await setProperty(ruleView, prop2, "flex");
+  info("Wait for change to be tracked");
+  await onTrackChange;
+
+  // Ensure changes to the second declaration were tracked after removing the first one.
+  await waitFor(
+    () => getRemovedDeclarations(doc).length == 2,
+    "Two declarations should have been tracked as removed"
+  );
+  await waitFor(() => {
+    const addDecl = getAddedDeclarations(doc);
+    return addDecl.length == 1 && addDecl[0].value == "flex";
+  }, "One declaration should have been tracked as added, and the added declaration to have updated property value");
+});
diff --git a/devtools/client/inspector/changes/test/browser_changes_declaration_remove_disabled.js b/devtools/client/inspector/changes/test/browser_changes_declaration_remove_disabled.js
new file mode 100644
index 0000000000..421d22cbae
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_declaration_remove_disabled.js
@@ -0,0 +1,107 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that disabling a CSS declaration and then removing it from the Rule view
+// is tracked as removed only once. Toggling leftover declarations should not introduce
+// duplicate changes.
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+      color: red;
+      background: black;
+    }
+  </style>
+  <div></div>
+`;
+
+add_task(async function () {
+  await pushPref("devtools.inspector.rule-view.focusNextOnEnter", false);
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const { document: doc, store } = selectChangesView(inspector);
+
+  await selectNode("div", inspector);
+  const prop1 = getTextProperty(ruleView, 1, { color: "red" });
+  const prop2 = getTextProperty(ruleView, 1, { background: "black" });
+
+  info("Using the second declaration");
+  await testRemoveValue(ruleView, store, doc, prop2);
+  info("Using the first declaration");
+  await testToggle(ruleView, store, doc, prop1);
+  info("Using the first declaration");
+  await testRemoveName(ruleView, store, doc, prop1);
+});
+
+async function testRemoveValue(ruleView, store, doc, prop) {
+  info("Test removing disabled declaration by clearing its property value.");
+  let onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Disable the declaration");
+  await togglePropStatus(ruleView, prop);
+  info("Wait for change to be tracked");
+  await onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Remove the disabled declaration by clearing its value");
+  await setProperty(ruleView, prop, null);
+  await onTrackChange;
+
+  const removeDecl = getRemovedDeclarations(doc);
+  is(removeDecl.length, 1, "Only one declaration tracked as removed");
+}
+
+async function testToggle(ruleView, store, doc, prop) {
+  info(
+    "Test toggling leftover declaration off and on will not track extra changes."
+  );
+  let onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Disable the declaration");
+  await togglePropStatus(ruleView, prop);
+  await onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Re-enable the declaration");
+  await togglePropStatus(ruleView, prop);
+  await onTrackChange;
+
+  await waitFor(
+    () => getRemovedDeclarations(doc).length == 1,
+    "Still just one declaration tracked as removed"
+  );
+}
+
+async function testRemoveName(ruleView, store, doc, prop) {
+  info("Test removing disabled declaration by clearing its property name.");
+  let onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Disable the declaration");
+  await togglePropStatus(ruleView, prop);
+  await onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Remove the disabled declaration by clearing its name");
+  await removeProperty(ruleView, prop);
+  await onTrackChange;
+
+  info(`Expecting two declarations removed:
+  - one removed by its value in the other test
+  - one removed by its name from this test
+  `);
+
+  await waitFor(
+    () => getRemovedDeclarations(doc).length == 2,
+    "Two declarations tracked as removed"
+  );
+  const removeDecl = getRemovedDeclarations(doc);
+  is(removeDecl[0].property, "background", "First declaration name correct");
+  is(removeDecl[0].value, "black", "First declaration value correct");
+  is(removeDecl[1].property, "color", "Second declaration name correct");
+  is(removeDecl[1].value, "red", "Second declaration value correct");
+}
diff --git a/devtools/client/inspector/changes/test/browser_changes_declaration_rename.js b/devtools/client/inspector/changes/test/browser_changes_declaration_rename.js
new file mode 100644
index 0000000000..245ce50121
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_declaration_rename.js
@@ -0,0 +1,68 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that renaming the property of a CSS declaration in the Rule view is tracked.
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+      color: red;
+    }
+  </style>
+  <div></div>
+`;
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const { document: doc, store } = selectChangesView(inspector);
+
+  await selectNode("div", inspector);
+  const prop = getTextProperty(ruleView, 1, { color: "red" });
+
+  let onTrackChange;
+
+  const oldPropertyName = "color";
+  const newPropertyName = "background-color";
+
+  info(`Rename the CSS declaration name to ${newPropertyName}`);
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  await renameProperty(ruleView, prop, newPropertyName);
+  info("Wait for the change to be tracked");
+  await onTrackChange;
+
+  const removeDecl = getRemovedDeclarations(doc);
+  const addDecl = getAddedDeclarations(doc);
+
+  is(removeDecl.length, 1, "One declaration tracked as removed");
+  is(
+    removeDecl[0].property,
+    oldPropertyName,
+    `Removed declaration has old property name: ${oldPropertyName}`
+  );
+  is(addDecl.length, 1, "One declaration tracked as added");
+  is(
+    addDecl[0].property,
+    newPropertyName,
+    `Added declaration has new property name: ${newPropertyName}`
+  );
+
+  info(
+    `Reverting the CSS declaration name to ${oldPropertyName} should clear changes.`
+  );
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  await renameProperty(ruleView, prop, oldPropertyName);
+  info("Wait for the change to be tracked");
+  await onTrackChange;
+
+  await waitFor(
+    () => !getRemovedDeclarations(doc).length,
+    "No declaration tracked as removed"
+  );
+  await waitFor(
+    () => !getAddedDeclarations(doc).length,
+    "No declaration tracked as added"
+  );
+});
diff --git a/devtools/client/inspector/changes/test/browser_changes_nested_rules.js b/devtools/client/inspector/changes/test/browser_changes_nested_rules.js
new file mode 100644
index 0000000000..789d88fdda
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_nested_rules.js
@@ -0,0 +1,189 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that the Changes panel works with nested rules.
+
+// Declare rule individually so we can use them for the assertions as well
+// In the end, we should have nested rule looking like:
+// - @media screen and (height > 5px) {
+// -- @layer myLayer {
+// --- @container myContainer (width > 10px) {
+// ----- div {
+// ------- & > span { … }
+// ------- .mySpan {
+// --------- &:not(:focus) {
+
+const spanNotFocusedRule = `&:not(:focus) {
+  text-decoration: underline;
+}`;
+
+const spanClassRule = `.mySpan {
+  font-weight: bold;
+  ${spanNotFocusedRule}
+}`;
+
+const spanRule = `& > span {
+  outline: 1px solid gold;
+}`;
+
+const divRule = `div {
+  color: tomato;
+  ${spanRule}
+  ${spanClassRule}
+}`;
+
+const containerRule = `@container myContainer (width > 10px) {
+  /* in container */
+  ${divRule}
+}`;
+const layerRule = `@layer myLayer {
+  /* in layer */
+  ${containerRule}
+}`;
+const mediaRule = `@media screen and (height > 5px) {
+  /* in media */
+    ${layerRule}
+}`;
+
+const TEST_URI = `
+  <style>
+  body {
+    container: myContainer / inline-size
+  }
+  ${mediaRule}
+  </style>
+  <div>hello <span class="mySpan">world</span></div>
+`;
+
+const applyModificationAfterDivPropertyChange = ruleText =>
+  ruleText.replace("tomato", "cyan");
+
+const EXPECTED_AFTER_DIV_PROP_CHANGE = [
+  {
+    text: "@media screen and (height > 5px) {",
+    copyRuleClipboard: applyModificationAfterDivPropertyChange(mediaRule),
+  },
+  {
+    text: "@layer myLayer {",
+    copyRuleClipboard: applyModificationAfterDivPropertyChange(layerRule),
+  },
+  {
+    text: "@container myContainer (width > 10px) {",
+    copyRuleClipboard: applyModificationAfterDivPropertyChange(containerRule),
+  },
+  {
+    text: "div {",
+    copyRuleClipboard: applyModificationAfterDivPropertyChange(divRule),
+  },
+];
+
+const applyModificationAfterSpanPropertiesChange = ruleText =>
+  ruleText
+    .replace("1px solid gold", "4px solid gold")
+    .replace("bold", "bolder")
+    .replace("underline", "underline dotted");
+
+const EXPECTED_AFTER_SPAN_PROP_CHANGES = EXPECTED_AFTER_DIV_PROP_CHANGE.map(
+  expected => ({
+    ...expected,
+    copyRuleClipboard: applyModificationAfterSpanPropertiesChange(
+      expected.copyRuleClipboard
+    ),
+  })
+).concat([
+  {
+    text: ".mySpan {",
+    copyRuleClipboard:
+      applyModificationAfterSpanPropertiesChange(spanClassRule),
+  },
+  {
+    text: "&:not(:focus) {",
+    copyRuleClipboard:
+      applyModificationAfterSpanPropertiesChange(spanNotFocusedRule),
+  },
+  {
+    text: "& > span {",
+    copyRuleClipboard: applyModificationAfterSpanPropertiesChange(spanRule),
+  },
+]);
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const changesView = selectChangesView(inspector);
+  const { document: panelDoc, store } = changesView;
+  const panel = panelDoc.querySelector("#sidebar-panel-changes");
+
+  await selectNode("div", inspector);
+  let onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  await updateDeclaration(ruleView, 1, { color: "tomato" }, { color: "cyan" });
+  await onTrackChange;
+
+  await assertSelectors(panel, EXPECTED_AFTER_DIV_PROP_CHANGE);
+
+  await selectNode(".mySpan", inspector);
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  await updateDeclaration(
+    ruleView,
+    1,
+    { "text-decoration": "underline" },
+    { "text-decoration": "underline dotted" }
+  );
+  await onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  await updateDeclaration(
+    ruleView,
+    2,
+    { "font-weight": "bold" },
+    { "font-weight": "bolder" }
+  );
+  await onTrackChange;
+
+  onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  await updateDeclaration(
+    ruleView,
+    3,
+    { outline: "1px solid gold" },
+    { outline: "4px solid gold" }
+  );
+  await onTrackChange;
+
+  await assertSelectors(panel, EXPECTED_AFTER_SPAN_PROP_CHANGES);
+});
+
+async function assertSelectors(panel, expected) {
+  const selectorsEl = getSelectors(panel);
+
+  is(
+    selectorsEl.length,
+    expected.length,
+    "Got the expected number of selectors item"
+  );
+
+  for (let i = 0; i < expected.length; i++) {
+    const selectorEl = selectorsEl[i];
+    const expectedItem = expected[i];
+
+    is(
+      selectorEl.innerText,
+      expectedItem.text,
+      `Got expected selector text at index ${i}`
+    );
+    info(`Click the Copy Rule button for the "${expectedItem.text}" rule`);
+    const button = selectorEl
+      .closest(".changes__rule")
+      .querySelector(".changes__copy-rule-button");
+    await waitForClipboardPromise(
+      () => button.click(),
+      () => checkClipboardData(expectedItem.copyRuleClipboard)
+    );
+  }
+}
+
+function checkClipboardData(expected) {
+  const actual = SpecialPowers.getClipboardData("text/plain");
+  return actual.trim() === expected.trim();
+}
diff --git a/devtools/client/inspector/changes/test/browser_changes_rule_add.js b/devtools/client/inspector/changes/test/browser_changes_rule_add.js
new file mode 100644
index 0000000000..215f1f3605
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_rule_add.js
@@ -0,0 +1,64 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that adding a new CSS rule in the Rules view is tracked in the Changes panel.
+// Renaming the selector of the new rule should overwrite the tracked selector.
+
+const TEST_URI = `
+  <div></div>
+`;
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const { document: doc, store } = selectChangesView(inspector);
+  const panel = doc.querySelector("#sidebar-panel-changes");
+
+  await selectNode("div", inspector);
+  await testTrackAddNewRule(store, inspector, ruleView, panel);
+  await testTrackRenameNewRule(store, inspector, ruleView, panel);
+});
+
+async function testTrackAddNewRule(store, inspector, ruleView, panel) {
+  const onTrackChange = waitForDispatch(store, "TRACK_CHANGE");
+  info("Adding a new CSS rule in the Rule view");
+  await addNewRule(inspector, ruleView);
+  info("Pressing escape to leave the editor");
+  EventUtils.synthesizeKey("KEY_Escape");
+  info("Waiting for changes to be tracked");
+  await onTrackChange;
+
+  const addedSelectors = getAddedSelectors(panel);
+  const removedSelectors = getRemovedSelectors(panel);
+  is(addedSelectors.length, 1, "One selector was tracked as added");
+  is(addedSelectors.item(0).title, "div", "New rule's has DIV selector");
+  is(removedSelectors.length, 0, "No selectors tracked as removed");
+}
+
+async function testTrackRenameNewRule(store, inspector, ruleView, panel) {
+  info("Focusing the first rule's selector name in the Rule view");
+  const ruleEditor = getRuleViewRuleEditor(ruleView, 1);
+  const editor = await focusEditableField(ruleView, ruleEditor.selectorText);
+  info("Entering a new selector name");
+  editor.input.value = ".test";
+
+  // Expect two "TRACK_CHANGE" actions: one for removal, one for addition.
+  const onTrackChange = waitForDispatch(store, "TRACK_CHANGE", 2);
+  const onRuleViewChanged = once(ruleView, "ruleview-changed");
+  EventUtils.synthesizeKey("KEY_Enter");
+  await onRuleViewChanged;
+  info("Waiting for changes to be tracked");
+  await onTrackChange;
+
+  const addedSelectors = getAddedSelectors(panel);
+  const removedSelectors = getRemovedSelectors(panel);
+  is(addedSelectors.length, 1, "One selector was tracked as added");
+  is(
+    addedSelectors.item(0).title,
+    ".test",
+    "New rule's selector was updated in place."
+  );
+  is(removedSelectors.length, 0, "No selectors tracked as removed");
+}
diff --git a/devtools/client/inspector/changes/test/browser_changes_rule_selector.js b/devtools/client/inspector/changes/test/browser_changes_rule_selector.js
new file mode 100644
index 0000000000..20d3fba654
--- /dev/null
+++ b/devtools/client/inspector/changes/test/browser_changes_rule_selector.js
@@ -0,0 +1,60 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that renaming the selector of a CSS rule is tracked.
+// Expect a selector removal followed by a selector addition and no changed declarations
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+      color: red;
+    }
+  </style>
+  <div></div>
+`;
+
+add_task(async function () {
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, view: ruleView } = await openRuleView();
+  const { document: doc, store } = selectChangesView(inspector);
+  const panel = doc.querySelector("#sidebar-panel-changes");
+
+  await selectNode("div", inspector);
+  const ruleEditor = getRuleViewRuleEditor(ruleView, 1);
+
+  info("Focusing the first rule's selector name in the Rule view");
+  const editor = await focusEditableField(ruleView, ruleEditor.selectorText);
+  info("Entering a new selector name");
+  editor.input.value = ".test";
+
+  // Expect two "TRACK_CHANGE" actions: one for removal, one for addition.
+  const onTrackChange = waitForDispatch(store, "TRACK_CHANGE", 2);
+  const onRuleViewChanged = once(ruleView, "ruleview-changed");
+  info("Pressing Enter key to commit the change");
+  EventUtils.synthesizeKey("KEY_Enter");
+  info("Waiting for rule view to update");
+  await onRuleViewChanged;
+  info("Wait for the change to be tracked");
+  await onTrackChange;
+
+  const rules = panel.querySelectorAll(".changes__rule");
+  is(rules.length, 1, "One rule was tracked as changed");
+
+  info("Expect old selector to be removed and new selector to be added");
+  const addedSelectors = getAddedSelectors(panel);
+  const removedSelectors = getRemovedSelectors(panel);
+  is(addedSelectors.length, 1, "One new selector was tracked as added");
+  is(addedSelectors.item(0).title, ".test", "New selector is correct");
+  is(removedSelectors.length, 1, "One old selector was tracked as removed");
+  is(removedSelectors.item(0).title, "div", "Old selector is correct");
+
+  info(
+    "Expect no declarations to have been added or removed during selector change"
+  );
+  const removeDecl = getRemovedDeclarations(doc, rules.item(0));
+  is(removeDecl.length, 0, "No declarations removed");
+  const addDecl = getAddedDeclarations(doc, rules.item(0));
+  is(addDecl.length, 0, "No declarations added");
+});
diff --git a/devtools/client/inspector/changes/test/head.js b/devtools/client/inspector/changes/test/head.js
new file mode 100644
index 0000000000..b45af3ba47
--- /dev/null
+++ b/devtools/client/inspector/changes/test/head.js
@@ -0,0 +1,93 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+/* eslint no-unused-vars: [2, {"vars": "local"}] */
+
+"use strict";
+
+// Load the Rule view's test/head.js to make use of its helpers.
+// It loads inspector/test/head.js which itself loads inspector/test/shared-head.js
+Services.scriptloader.loadSubScript(
+  "chrome://mochitests/content/browser/devtools/client/inspector/rules/test/head.js",
+  this
+);
+
+// Ensure the three-pane mode is enabled before running the tests.
+Services.prefs.setBoolPref("devtools.inspector.three-pane-enabled", true);
+
+registerCleanupFunction(() => {
+  Services.prefs.clearUserPref("devtools.inspector.three-pane-enabled");
+});
+
+/**
+ * Get an array of objects with property/value pairs of the CSS declarations rendered
+ * in the Changes panel.
+ *
+ * @param  {Document} panelDoc
+ *         Host document of the Changes panel.
+ * @param  {String} selector
+ *         Optional selector to filter rendered declaration DOM elements.
+ *         One of ".diff-remove" or ".diff-add".
+ *         If omitted, all declarations will be returned.
+ * @param  {DOMNode} containerNode
+ *         Optional element to restrict results to declaration DOM elements which are
+ *         descendants of this container node.
+ *         If omitted, all declarations will be returned
+ * @return {Array}
+ */
+function getDeclarations(panelDoc, selector = "", containerNode = null) {
+  const els = panelDoc.querySelectorAll(`.changes__declaration${selector}`);
+
+  return [...els]
+    .filter(el => {
+      return containerNode ? containerNode.contains(el) : true;
+    })
+    .map(el => {
+      return {
+        property: el.querySelector(".changes__declaration-name").textContent,
+        value: el.querySelector(".changes__declaration-value").textContent,
+        element: el,
+      };
+    });
+}
+
+function getAddedDeclarations(panelDoc, containerNode) {
+  return getDeclarations(panelDoc, ".diff-add", containerNode);
+}
+
+function getRemovedDeclarations(panelDoc, containerNode) {
+  return getDeclarations(panelDoc, ".diff-remove", containerNode);
+}
+
+/**
+ * Get an array of DOM elements for the CSS selectors rendered in the Changes panel.
+ *
+ * @param  {Document} panelDoc
+ *         Host document of the Changes panel.
+ * @param  {String} selector
+ *         Optional selector to filter rendered selector DOM elements.
+ *         One of ".diff-remove" or ".diff-add".
+ *         If omitted, all selectors will be returned.
+ * @return {Array}
+ */
+function getSelectors(panelDoc, selector = "") {
+  return panelDoc.querySelectorAll(`.changes__selector${selector}`);
+}
+
+function getAddedSelectors(panelDoc) {
+  return getSelectors(panelDoc, ".diff-add");
+}
+
+function getRemovedSelectors(panelDoc) {
+  return getSelectors(panelDoc, ".diff-remove");
+}
+
+async function getChangesContextMenu(changesView, element) {
+  const onContextMenu = changesView.contextMenu.once("open");
+  info(`Trigger context menu for element: ${element}`);
+  synthesizeContextMenuEvent(element);
+  info(`Wait for context menu to show`);
+  await onContextMenu;
+
+  return changesView.contextMenu;
+}
diff --git a/devtools/client/inspector/changes/test/xpcshell/.eslintrc.js b/devtools/client/inspector/changes/test/xpcshell/.eslintrc.js
new file mode 100644
index 0000000000..86bd54c245
--- /dev/null
+++ b/devtools/client/inspector/changes/test/xpcshell/.eslintrc.js
@@ -0,0 +1,6 @@
+"use strict";
+
+module.exports = {
+  // Extend from the common devtools xpcshell eslintrc config.
+  extends: "../../../../../.eslintrc.xpcshell.js",
+};
diff --git a/devtools/client/inspector/changes/test/xpcshell/head.js b/devtools/client/inspector/changes/test/xpcshell/head.js
new file mode 100644
index 0000000000..f08a79dd71
--- /dev/null
+++ b/devtools/client/inspector/changes/test/xpcshell/head.js
@@ -0,0 +1,8 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { require } = ChromeUtils.importESModule(
+  "resource://devtools/shared/loader/Loader.sys.mjs"
+);
diff --git a/devtools/client/inspector/changes/test/xpcshell/mocks.js b/devtools/client/inspector/changes/test/xpcshell/mocks.js
new file mode 100644
index 0000000000..52f175beb8
--- /dev/null
+++ b/devtools/client/inspector/changes/test/xpcshell/mocks.js
@@ -0,0 +1,67 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+/* eslint-disable comma-dangle */
+
+"use strict";
+
+/**
+ * Snapshot of the Redux state for the Changes panel.
+ *
+ * It corresponds to the tracking of a single property value change (background-color)
+ * within a deeply nested CSS at-rule structure from an inline stylesheet:
+ *
+ * @media (min-width: 50em) {
+ *   @supports (display: grid) {
+ *     body {
+ *       - background-color: royalblue;
+ *       + background-color: red;
+ *     }
+ *   }
+ * }
+ */
+module.exports.CHANGES_STATE = {
+  source1: {
+    type: "inline",
+    href: "http://localhost:5000/at-rules-nested.html",
+    id: "source1",
+    index: 0,
+    isFramed: false,
+    rules: {
+      rule1: {
+        selectors: ["@media (min-width: 50em)"],
+        ruleId: "rule1",
+        add: [],
+        remove: [],
+        children: ["rule2"],
+      },
+      rule2: {
+        selectors: ["@supports (display: grid)"],
+        ruleId: "rule2",
+        add: [],
+        remove: [],
+        children: ["rule3"],
+        parent: "rule1",
+      },
+      rule3: {
+        selectors: ["body"],
+        ruleId: "rule3",
+        add: [
+          {
+            property: "background-color",
+            value: "red",
+            index: 0,
+          },
+        ],
+        remove: [
+          {
+            property: "background-color",
+            value: "royalblue",
+            index: 0,
+          },
+        ],
+        children: [],
+        parent: "rule2",
+      },
+    },
+  },
+};
diff --git a/devtools/client/inspector/changes/test/xpcshell/test_changes_stylesheet.js b/devtools/client/inspector/changes/test/xpcshell/test_changes_stylesheet.js
new file mode 100644
index 0000000000..33a64cfbcb
--- /dev/null
+++ b/devtools/client/inspector/changes/test/xpcshell/test_changes_stylesheet.js
@@ -0,0 +1,60 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Check that getChangesStylesheet() serializes tracked changes from nested CSS rules
+// into the expected stylesheet format.
+
+const {
+  getChangesStylesheet,
+} = require("resource://devtools/client/inspector/changes/selectors/changes.js");
+
+const { CHANGES_STATE } = require("resource://test/mocks");
+
+// Wrap multi-line string in backticks to ensure exact check in test, including new lines.
+const STYLESHEET_FOR_ANCESTOR = `
+/* Inline #0 | http://localhost:5000/at-rules-nested.html */
+
+@media (min-width: 50em) {
+  @supports (display: grid) {
+    body {
+      /* background-color: royalblue; */
+      background-color: red;
+    }
+  }
+}
+`;
+
+// Wrap multi-line string in backticks to ensure exact check in test, including new lines.
+const STYLESHEET_FOR_DESCENDANT = `
+/* Inline #0 | http://localhost:5000/at-rules-nested.html */
+
+body {
+  /* background-color: royalblue; */
+  background-color: red;
+}
+`;
+
+add_test(() => {
+  info(
+    "Check stylesheet generated for the first ancestor in the CSS rule tree."
+  );
+  equal(
+    getChangesStylesheet(CHANGES_STATE),
+    STYLESHEET_FOR_ANCESTOR,
+    "Stylesheet includes all ancestors."
+  );
+
+  info(
+    "Check stylesheet generated for the last descendant in the CSS rule tree."
+  );
+  const filter = { sourceIds: ["source1"], ruleIds: ["rule3"] };
+  equal(
+    getChangesStylesheet(CHANGES_STATE, filter),
+    STYLESHEET_FOR_DESCENDANT,
+    "Stylesheet includes just descendant."
+  );
+
+  run_next_test();
+});
diff --git a/devtools/client/inspector/changes/test/xpcshell/xpcshell.toml b/devtools/client/inspector/changes/test/xpcshell/xpcshell.toml
new file mode 100644
index 0000000000..2edc192d20
--- /dev/null
+++ b/devtools/client/inspector/changes/test/xpcshell/xpcshell.toml
@@ -0,0 +1,7 @@
+[DEFAULT]
+tags = "devtools"
+firefox-appdir = "browser"
+head = "head.js"
+support-files = ["./mocks.js"]
+
+["test_changes_stylesheet.js"]
diff --git a/devtools/client/inspector/changes/utils/changes-utils.js b/devtools/client/inspector/changes/utils/changes-utils.js
new file mode 100644
index 0000000000..3e8505dc17
--- /dev/null
+++ b/devtools/client/inspector/changes/utils/changes-utils.js
@@ -0,0 +1,44 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {
+  getFormatStr,
+  getStr,
+} = require("resource://devtools/client/inspector/changes/utils/l10n.js");
+
+/**
+ * Get a human-friendly style source path to display in the Changes panel.
+ * For element inline styles, return a string indicating that.
+ * For inline stylesheets, return a string indicating that plus the stylesheet's index.
+ * For URLs, return just the stylesheet filename.
+ *
+ * @param {Object} source
+ *        Information about the style source. Contains:
+ *        - type: {String} One of "element" or "stylesheet"
+ *        - href: {String|null} Stylesheet URL or document URL for elmeent inline styles
+ *        - index: {Number} Position of the stylesheet in its document's stylesheet list.
+ * @return {String}
+ */
+function getSourceForDisplay(source) {
+  let href;
+
+  switch (source.type) {
+    case "element":
+      href = getStr("changes.elementStyleLabel");
+      break;
+    case "inline":
+      href = getFormatStr("changes.inlineStyleSheetLabel", `#${source.index}`);
+      break;
+    case "stylesheet":
+      const url = new URL(source.href);
+      href = url.pathname.substring(url.pathname.lastIndexOf("/") + 1);
+      break;
+  }
+
+  return href;
+}
+
+module.exports.getSourceForDisplay = getSourceForDisplay;
diff --git a/devtools/client/inspector/changes/utils/l10n.js b/devtools/client/inspector/changes/utils/l10n.js
new file mode 100644
index 0000000000..693a1ec3cf
--- /dev/null
+++ b/devtools/client/inspector/changes/utils/l10n.js
@@ -0,0 +1,15 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const { LocalizationHelper } = require("resource://devtools/shared/l10n.js");
+const L10N = new LocalizationHelper(
+  "devtools/client/locales/changes.properties"
+);
+
+module.exports = {
+  getStr: (...args) => L10N.getStr(...args),
+  getFormatStr: (...args) => L10N.getFormatStr(...args),
+};
diff --git a/devtools/client/inspector/changes/utils/moz.build b/devtools/client/inspector/changes/utils/moz.build
new file mode 100644
index 0000000000..155752e0d8
--- /dev/null
+++ b/devtools/client/inspector/changes/utils/moz.build
@@ -0,0 +1,10 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+DevToolsModules(
+    "changes-utils.js",
+    "l10n.js",
+)
-- 
cgit v1.2.3