Detect main window(which has most tabs) and sub(second most tabs).

detect_main_window_on_chrome_extension.js

let mainId, subId;

const detectMainWinodows = () => {
  chrome.windows.getAll({populate: true, windowTypes: ['normal']}, (windows) => {
    if (windows.length == 1) return;

    const sortedWindowIdArray = windows.sort((a, b) => b.tabs.length - a.tabs.length).map((win) => win.id);

    mainId = sortedWindowIdArray[0];
    subId = sortedWindowIdArray[1];
  }
  )
}

chrome.runtime.onStartup.addListener(detectMainWinodows);
chrome.runtime.onInstalled.addListener((details) => {
  if(details.reason == "install") detectMainWinodows();
});
chrome.windows.onCreated.addListener(detectMainWinodows);
chrome.windows.onRemoved.addListener(detectMainWinodows);