Thanks for your suggestion!
It may have seemed like a bug, but it actually is a hack:-)

This "hidden rectangle" is used to hide cutted polygon borders.
For instance, see

data(Europe)
qtm(Europe, fill="income_grp") + tm_layout(draw.frame=FALSE) 

The polygons of all countries that cross the frame are cut. However, these cut lines, that lie exactly on the frame border, are artificial borders, so therefore I hide them.

Strange that this line is shown through the legend. That's what I've found too if the legend is positioned across the frame:

qtm(Europe, fill="income_grp") +
tm_layout(draw.frame=FALSE, legend.position=c(.10,.9))

I'll try to fix this soon.

Thanks for your reply!
Maybe you could change the plot order, so the legend will be plotted over the rectangle? The same approach could be used with the frame: always plot the white rectangle and just overplot it with a colored frame if draw.frame=T.

My thoughts exactly, thanks!