The Doc says if you want to create a panel window ,you should use
chrome.windows.create({...,type:"panel"}, function callback)
At the same time, it says "Specifies what type of browser window to create. The 'panel' type creates a popup unless the '--enable-panels' flag is set."
So you should go to "chrome://flags" to change it manually. But Google Hangouts can create the panel without doing that. How does it do that?
The Google Hangouts extension (formerly called "Chat for Google") is explicitly whitelisted in the source code, which allows it to use the panel feature even when
--enable-panels
flag is unset: