Capture events in other applications

[C R]

I would like to write a program that sits in the background and captures
double-click events (or function keys) in other applications, then does
something with them or not depending on which application they happen
in.

Specifically, if they occur in a web browser, I'd like to grab the page
source in the browser and present it in a different format in my own
application (then exit back to the browser when the user is done).

Any pointers on how to go about this?

Thanks in advance!

CR

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