Copy text from rectangle selection in Chrome

A picture is worth a thousand words :

In Rietveld web diff tool I want to copy code from one of the buffer, but text selection with the mouse spreads over the other buffer when selecting multiple lines.

I wish I could draw a rectangle on screen and all text contained in that bounding box be pasted in my clipboard. Is there an extension for that ?

Edit:

Here is the kind of result I'd like to achieve, say I want to copy the right buffer :

The content you wish to separate is wrapped in a table.

If you copy/paste it into a google sheets document, you will get the left and right side on separate columns, and can simply mark one of them to copy it's content into the text editor of your choice.

Here's a working example (make a copy of the sheet and change the URL in D2 to load another page)

I hope it helps.