chrome-extensions-samples/api-samples/windows/background.js

// Copyright 2023 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

async function start() {
  const current = await chrome.windows.getCurrent();

  const allTabs = await chrome.tabs.query({});
  allTabs.forEach((tab) => {
    if (tab.windowId != current.id) {
      chrome.tabs.move(tab.id, {
        windowId: current.id,
        index: tab.index
      });
    }
  });
}

// Set up a click handler so that we can merge all the windows.
chrome.action.onClicked.addListener(start);