Triple-click and drag to select full lines

[Chris Picton]

  Hi all

My preferred behavious for triple-click and drag is for the initial 
triple-click to select a full line, and a subsequent drag of the mouse 
up or down to keep the initial entire line selected and continue 
selecting entire lines.

Currently, the initial triple-click does select a full line, but the 
subsequent drag unhilights part of the line and then continues selecting 
words only.

Can anyone please guide me in the correct direction to modify the 
existing, or create a new plugin to handle this scenario.

Many Thanks

Chris