When using the mouse to mark some text, the lines get reduced to the
actually used length - ie. the markup doesn't extend to the right border of
the terminal window.
But this isn't so for the first line - I guess the reason is the "[X/Y]"
hint shown during markup.
To reproduce: Have some output in a tmux window, and press the mouse button
in the first line. When moving the mouse down, the 2nd and later lines only
have the space marked up that was "used" - but the first line extends to
the right border.