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diff --git a/devtools/client/debugger/src/reducers/sources-content.js b/devtools/client/debugger/src/reducers/sources-content.js
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+++ b/devtools/client/debugger/src/reducers/sources-content.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/>. */
+
+/**
+ * Sources content reducer.
+ *
+ * This store the textual content for each source.
+ */
+
+import { pending, fulfilled, rejected } from "../utils/async-value";
+
+export function initialSourcesContentState() {
+  return {
+    /**
+     * Text content of all the original sources.
+     * This is large data, so this is only fetched on-demand for a subset of sources.
+     * This state attribute is mutable in order to avoid cloning this possibly large map
+     * on each new source. But selectors are never based on the map. Instead they only
+     * query elements of the map.
+     *
+     * Map(source id => AsyncValue<String>)
+     */
+    mutableOriginalSourceTextContentMapBySourceId: new Map(),
+
+    /**
+     * Text content of all the generated sources.
+     *
+     * Map(source actor is => AsyncValue<String>)
+     */
+    mutableGeneratedSourceTextContentMapBySourceActorId: new Map(),
+
+    /**
+     * Incremental number that is bumped each time we navigate to a new page.
+     *
+     * This is used to better handle async race condition where we mix previous page data
+     * with the new page. As sources are keyed by URL we may easily conflate the two page loads data.
+     */
+    epoch: 1,
+  };
+}
+
+function update(state = initialSourcesContentState(), action) {
+  switch (action.type) {
+    case "LOAD_ORIGINAL_SOURCE_TEXT":
+      if (!action.sourceId) {
+        throw new Error("No source id found.");
+      }
+      return updateSourceTextContent(state, action);
+
+    case "LOAD_GENERATED_SOURCE_TEXT":
+      if (!action.sourceActorId) {
+        throw new Error("No source actor id found.");
+      }
+      return updateSourceTextContent(state, action);
+
+    case "REMOVE_THREAD":
+      return removeThread(state, action);
+  }
+
+  return state;
+}
+
+/*
+ * Update a source's loaded text content.
+ */
+function updateSourceTextContent(state, action) {
+  // If there was a navigation between the time the action was started and
+  // completed, we don't want to update the store.
+  if (action.epoch !== state.epoch) {
+    return state;
+  }
+
+  let content;
+  if (action.status === "start") {
+    content = pending();
+  } else if (action.status === "error") {
+    content = rejected(action.error);
+  } else if (typeof action.value.text === "string") {
+    content = fulfilled({
+      type: "text",
+      value: action.value.text,
+      contentType: action.value.contentType,
+    });
+  } else {
+    content = fulfilled({
+      type: "wasm",
+      value: action.value.text,
+    });
+  }
+
+  if (action.sourceId && action.sourceActorId) {
+    throw new Error(
+      "Both the source id and the source actor should not exist at the same time"
+    );
+  }
+
+  if (action.sourceId) {
+    state.mutableOriginalSourceTextContentMapBySourceId.set(
+      action.sourceId,
+      content
+    );
+  }
+
+  if (action.sourceActorId) {
+    state.mutableGeneratedSourceTextContentMapBySourceActorId.set(
+      action.sourceActorId,
+      content
+    );
+  }
+
+  return {
+    ...state,
+  };
+}
+
+function removeThread(state, action) {
+  const originalSizeBefore =
+    state.mutableOriginalSourceTextContentMapBySourceId.size;
+  for (const source of action.sources) {
+    state.mutableOriginalSourceTextContentMapBySourceId.delete(source.id);
+  }
+  const generatedSizeBefore =
+    state.mutableGeneratedSourceTextContentMapBySourceActorId.size;
+  for (const actor of action.actors) {
+    state.mutableGeneratedSourceTextContentMapBySourceActorId.delete(actor.id);
+  }
+  if (
+    originalSizeBefore !=
+      state.mutableOriginalSourceTextContentMapBySourceId.size ||
+    generatedSizeBefore !=
+      state.mutableGeneratedSourceTextContentMapBySourceActorId.size
+  ) {
+    return { ...state };
+  }
+  return state;
+}
+
+export default update;