Problems when map at non-zero positions

Hi,

First i want to thank the developers of ModestMaps for the great piece of software.

I have encountered the following problem with ModestMap (AS3 version):

When positioning a Map instance at a non zero position on the stage, only the area that lies between the stage top-left and the width and height of the map responds to mouse-events. In other words: If the map is positioned at x=100 and y=100, the right and bottom 100 pixels do not respond to any mouse events. The ModestMapsSample application shows the same problem, but it's hard to detect, because it only affects 20 pixels (PADDING).
The problem semms to lie in the buildMask method of the TileGrid class. If i omit the mask (or use scrollrect(0, 0, _width, _height), which is faster anyway) the problem does not occur.
I'm new to Actionscript and ModestMaps, so i do not understand, what's wrong with the mask. Btw., what's the mask good for? I haven't noticed any problems with omitting it completely.

thanks for any hints & greetings from wiesbaden, germany,
manfred