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diff --git a/devtools/client/debugger/src/reducers/tabs.js b/devtools/client/debugger/src/reducers/tabs.js
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+++ b/devtools/client/debugger/src/reducers/tabs.js
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+/* 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/>. */
+
+/**
+ * Tabs reducer
+ * @module reducers/tabs
+ */
+
+import { isOriginalId } from "devtools/client/shared/source-map-loader/index";
+
+import { isSimilarTab } from "../utils/tabs";
+
+export function initialTabState() {
+  return { tabs: [] };
+}
+
+function update(state = initialTabState(), action) {
+  switch (action.type) {
+    case "ADD_TAB":
+      return updateTabList(state, action.source, action.sourceActor);
+
+    case "MOVE_TAB":
+      return moveTabInList(state, action);
+
+    case "MOVE_TAB_BY_SOURCE_ID":
+      return moveTabInListBySourceId(state, action);
+
+    case "CLOSE_TAB":
+      return removeSourceFromTabList(state, action);
+
+    case "CLOSE_TABS":
+      return removeSourcesFromTabList(state, action);
+
+    case "ADD_ORIGINAL_SOURCES":
+      return addVisibleTabsForOriginalSources(
+        state,
+        action.originalSources,
+        action.generatedSourceActor
+      );
+
+    case "INSERT_SOURCE_ACTORS":
+      return addVisibleTabsForSourceActors(state, action.sourceActors);
+
+    case "REMOVE_THREAD": {
+      return resetTabsForThread(state, action.threadActorID);
+    }
+
+    default:
+      return state;
+  }
+}
+
+function matchesSource(tab, source) {
+  return tab.source?.id === source.id || matchesUrl(tab, source);
+}
+
+function matchesUrl(tab, source) {
+  return tab.url === source.url && tab.isOriginal == isOriginalId(source.id);
+}
+
+function addVisibleTabsForSourceActors(state, sourceActors) {
+  let changed = false;
+  // Lookups for tabs matching any source actor's URL
+  // and reference their source and sourceActor attribute
+  // so that the tab becomes visible.
+  const tabs = state.tabs.map(tab => {
+    const sourceActor = sourceActors.find(actor =>
+      matchesUrl(tab, actor.sourceObject)
+    );
+    if (!sourceActor) {
+      return tab;
+    }
+    changed = true;
+    return {
+      ...tab,
+      source: sourceActor.sourceObject,
+      sourceActor,
+    };
+  });
+
+  return changed ? { tabs } : state;
+}
+
+function addVisibleTabsForOriginalSources(
+  state,
+  sources,
+  generatedSourceActor
+) {
+  let changed = false;
+
+  // Lookups for tabs matching any source's URL
+  // and reference their source and sourceActor attribute
+  // so that the tab becomes visible.
+  const tabs = state.tabs.map(tab => {
+    const source = sources.find(s => matchesUrl(tab, s));
+    if (!source) {
+      return tab;
+    }
+    changed = true;
+    return {
+      ...tab,
+      source,
+      // All currently reported original sources are related to a single source actor
+      sourceActor: generatedSourceActor,
+    };
+  });
+
+  return changed ? { tabs } : state;
+}
+
+function removeSourceFromTabList(state, { source }) {
+  const newTabs = state.tabs.filter(tab => !matchesSource(tab, source));
+  if (newTabs.length == state.tabs.length) {
+    return state;
+  }
+  return { tabs: newTabs };
+}
+
+function removeSourcesFromTabList(state, { sources }) {
+  const newTabs = sources.reduce(
+    (tabList, source) => tabList.filter(tab => !matchesSource(tab, source)),
+    state.tabs
+  );
+  if (newTabs.length == state.tabs.length) {
+    return state;
+  }
+
+  return { tabs: newTabs };
+}
+
+function resetTabsForThread(state, threadActorID) {
+  let changed = false;
+  // Nullify source and sourceActor attributes of all tabs
+  // related to the given thread so that they become hidden.
+  //
+  // They may later be restored if a source matches their URL again.
+  // This is similar to persistTabs, but specific to a unique thread.
+  const tabs = state.tabs.map(tab => {
+    if (tab.sourceActor?.thread != threadActorID) {
+      return tab;
+    }
+    changed = true;
+    return {
+      ...tab,
+      source: null,
+      sourceActor: null,
+    };
+  });
+
+  return changed ? { tabs } : state;
+}
+
+/**
+ * Adds the new source to the tab list if it is not already there.
+ */
+function updateTabList(state, source, sourceActor) {
+  const { url } = source;
+  const isOriginal = isOriginalId(source.id);
+
+  let { tabs } = state;
+  // Set currentIndex to -1 for URL-less tabs so that they aren't
+  // filtered by isSimilarTab
+  const currentIndex = url
+    ? tabs.findIndex(tab => isSimilarTab(tab, url, isOriginal))
+    : -1;
+
+  if (currentIndex === -1) {
+    const newTab = {
+      url,
+      source,
+      isOriginal,
+      sourceActor,
+    };
+    // New tabs are added first in the list
+    tabs = [newTab, ...tabs];
+  } else {
+    return state;
+  }
+
+  return { ...state, tabs };
+}
+
+function moveTabInList(state, { url, tabIndex: newIndex }) {
+  const { tabs } = state;
+  const currentIndex = tabs.findIndex(tab => tab.url == url);
+  return moveTab(tabs, currentIndex, newIndex);
+}
+
+function moveTabInListBySourceId(state, { sourceId, tabIndex: newIndex }) {
+  const { tabs } = state;
+  const currentIndex = tabs.findIndex(tab => tab.source?.id == sourceId);
+  return moveTab(tabs, currentIndex, newIndex);
+}
+
+function moveTab(tabs, currentIndex, newIndex) {
+  const item = tabs[currentIndex];
+
+  const newTabs = Array.from(tabs);
+  // Remove the item from its current location
+  newTabs.splice(currentIndex, 1);
+  // And add it to the new one
+  newTabs.splice(newIndex, 0, item);
+
+  return { tabs: newTabs };
+}
+
+export default update;