Bad performance when drawing outside image edge if magnifier is on

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I've noticed it on my laptop which has very slow graphics, on Debian squeeze 
(6.0.6, fresh install) :
I open the magnifier view. I set the image size very small, like 10x10.
I choose the discontinuous freehand tool and a single pixel brush.
I draw just to the right of the image edge : the mouse cursor gets choppy and 
starts lagging behind until it catches up the mouse movement.
Strangely, with a user brush, this behavior doesn't happen.
It also doesn't happen if the image fills the same space, so it's not the 
normal "cumulated rectangle".

When I do the same test on my usual machine (12-year old desktop with AGP 2x 
videocard), it doesn't slow down, but CPU usage raises from ~10% to 40%.

Problem must be present in older versions of Grafx2 already.

Original issue reported on code.google.com by yrizoud on 8 Oct 2012 at 9:21