I am getting confused understanding the practices generally followed in the popular chrome extensions. I am trying to develop my own chrome extension and after going through the basic tutorial, I have a default popup page that opens whenever I click the extension icon near my address bar. So far so good! While checking the source codes of some good extensions installed in my chrome browser, I came to know, none of them uses the default_popup page but definitely invokes some javascripts through either the background page or content scripts. But the final behaviour as seen by the user is functionally like a popup at the upper right corner of the screen, though more presentable. Is there any reason for not using default_popup over using other mechanisms?
1 Answers
I think it really depends on what your app needs in terms of functionality and design. As there are no real reasons why you might want to choose one over the other. Most information can be passed from the page to the extension app and vice versa. Users expect a popup when they click on the button but injected popups are also supported and commonly used in Chrome, Firefox and Safari.
Pros/Cons:
If your extension depends on the page content then you can inject scripts that analyze the page and inject divs accordingly. You can send analyzed data back to the extension and open a popup but thats an additional step. If your extension has nothing to do with the specific page then you would be better off using a popup.
Popups close when you switch tabs or your browser loses focus. Injected popups need not.
Don't inject scripts and stylesheets into pages willy nilly. They interfere with a website's native js/css and also stuff injected by other externsions which is near impossible to fully account for.

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1Regarding your last point, this question is of note: http://stackoverflow.com/questions/12783217/how-to-really-isolate-stylesheets-in-the-google-chrome-extension – Xan Feb 26 '15 at 15:21