Thinking some more about what I was seeing in mapsource when mousing
around the Baltic map, I came to the conclusion that the edges of the
overview map bounding boxes where in the wrong place so I took a look
at mkgmap where it rounds those coords and came up with the attached
fix. No great theory here, I just dicked around until it worked as I
expected.

For the Baltic map, the mouse now works right up to each boundary in
both lat and lon directions whereas before it would only work right on
one side of each boundary.

Please try the patch, especially for maps that are W of Greenwich and S
of the equator and see if the overview boxes are right.

Cheers,

Mark