does what it says, though it doesn't really show a visual indicator of the region you're defining. i almost thought it was broken, but it turns out it doesnt check boxes on the same horizontal/vertical axis as the one you start dragging from. you can drag from outside the checkbox but it ends up selecting things like text etc as well. a little annoying.

it works perfect ... but when enabled the extension , the facebook messenger website can't load conversation list .... when disable the extention , facebook messenger works again.

Works exactly as described.