background.js

function createContextMenu() {
  chrome.contextMenus.removeAll(() => {
    chrome.contextMenus.create({
      id: "addAnnotation",
      title: "Create new annotation",
      contexts: ["selection"]
    });
  });
};

chrome.runtime.onInstalled.addListener(createContextMenu);
chrome.runtime.onStartup.addListener(createContextMenu);

chrome.contextMenus.onClicked.addListener((info, tab) => {
  if (info.menuItemId === "addAnnotation" && info.selectionText && info.selectionText.trim().length > 0) {
    chrome.scripting.executeScript({
      target: { tabId: tab.id },
      files: ['content.js']
    }, () => {
      chrome.tabs.sendMessage(tab.id, {
        action: "addAnnotation",
        selectedText: info.selectionText
      });
    });
  }
});