Make download button work

background.js

@@ -0,0 +1,161 @@
+const pendingResponses = new Map();
+const executionContext = {
+  mode: null,
+  windowId: null,
+  tabId: null
+};
+let contextReadyPromise = null;
+let contextReadyResolver = null;
+
+function createReadyPromise() {
+  contextReadyPromise = new Promise((resolve) => {
+    contextReadyResolver = resolve;
+  });
+}
+
+function resolveReadyPromise() {
+  if (contextReadyResolver) {
+    contextReadyResolver();
+    contextReadyResolver = null;
+  }
+  if (!contextReadyPromise) {
+    contextReadyPromise = Promise.resolve();
+  }
+}
+
+async function ensureDocumentContext() {
+  const hasOffscreenApi = Boolean(chrome.offscreen?.createDocument);
+  if (hasOffscreenApi) {
+    try {
+      if (chrome.offscreen.hasDocument) {
+        const exists = await chrome.offscreen.hasDocument();
+        if (exists) {
+          executionContext.mode = "offscreen";
+          if (!contextReadyPromise) {
+            contextReadyPromise = Promise.resolve();
+          }
+          await contextReadyPromise;
+          return;
+        }
+      }
+    } catch (error) {
+      // ignore and fall back
+    }
+
+    try {
+      createReadyPromise();
+      await chrome.offscreen.createDocument({
+        url: chrome.runtime.getURL("offscreen.html"),
+        reasons: ["DOM_PARSER"],
+        justification: "Process O'Reilly pages to build EPUB downloads"
+      });
+      executionContext.mode = "offscreen";
+      await contextReadyPromise;
+      return;
+    } catch (error) {
+      const alreadyExists = String(error?.message || "").includes("already exists");
+      if (!alreadyExists) {
+        console.warn("SafariBooks Downloader: Offscreen document failed, falling back to hidden window.", error);
+      } else {
+        executionContext.mode = "offscreen";
+        if (!contextReadyPromise) {
+          contextReadyPromise = Promise.resolve();
+        }
+        await contextReadyPromise;
+        return;
+      }
+    }
+  }
+
+  if (executionContext.mode === "window") {
+    if (executionContext.tabId != null) {
+      try {
+        await chrome.tabs.get(executionContext.tabId);
+        if (!contextReadyPromise) {
+          contextReadyPromise = Promise.resolve();
+        }
+        await contextReadyPromise;
+        return;
+      } catch (_) {
+        executionContext.tabId = null;
+      }
+    }
+  }
+
+  const popupUrl = chrome.runtime.getURL("offscreen.html#window");
+  const createdWindow = await chrome.windows.create({
+    url: popupUrl,
+    type: "popup",
+    focused: false,
+    width: 420,
+    height: 520
+  });
+
+  executionContext.mode = "window";
+  executionContext.windowId = createdWindow.id;
+  executionContext.tabId = createdWindow.tabs && createdWindow.tabs.length ? createdWindow.tabs[0].id : null;
+  createReadyPromise();
+  await contextReadyPromise;
+}
+
+chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
+  if (message?.type === "downloadBook") {
+    ensureDocumentContext()
+      .then(() => {
+        const requestId = crypto.randomUUID();
+        pendingResponses.set(requestId, sendResponse);
+        chrome.runtime
+          .sendMessage({
+            type: "offscreen-download",
+            requestId,
+            bookId: message.bookId,
+            options: {
+              theme: message.theme ?? "none",
+              kindle: Boolean(message.kindle)
+            }
+          })
+          .catch((error) => {
+            const responder = pendingResponses.get(requestId);
+            if (responder) {
+              responder({ ok: false, error: error.message });
+              pendingResponses.delete(requestId);
+            }
+          });
+      })
+      .catch((error) => {
+        sendResponse({ ok: false, error: error.message });
+      });
+    return true;
+  }
+
+  if (message?.type === "offscreen-ready") {
+    resolveReadyPromise();
+    return false;
+  }
+
+  if (message?.type === "offscreen-download-complete") {
+    const { requestId, ok, error } = message;
+    const responder = pendingResponses.get(requestId);
+    if (responder) {
+      responder({ ok, error });
+      pendingResponses.delete(requestId);
+    }
+
+    if (executionContext.mode === "window" && pendingResponses.size === 0 && executionContext.windowId != null) {
+      chrome.windows
+        .remove(executionContext.windowId)
+        .catch(() => {})
+        .finally(() => {
+          executionContext.windowId = null;
+          executionContext.tabId = null;
+          executionContext.mode = null;
+          contextReadyPromise = null;
+          contextReadyResolver = null;
+        });
+    }
+
+    return false;
+  }
+
+  return undefined;
+});