How to create a plugin or an extension app on a browser?

[111214]

Hello , I am currently trying to make an app that can be activate when you highlight a word on a browser , and after it is activated , it will retrieve information from a database . For example , there is a dictionary in the image below , which when you highlight something on a browser , there will be on a tag where you can click on and when you click it , it will retrieve data and pop up on the screen just like the image below . However I don't know how to do that or is it possible to do that.Thanks for helping .

[TIMAI2]

Try Taifun's Clipboard extension to capture the "copy event data" and return this to the app ?

[vknow360]

Currently
1.AI2's apps can't run in background.
2.You can't track other apps.

Also Clipboard api does provide any listener to listen clipboard changes, as far as I know.