I wrote some lib2geom pretty printers to help debug this problem. They
are in lib2geom's new 'tools' directory. To use them, add the
following to .gdbinit:
from lib2geom_gdb import register_lib2geom_printers
Using them, I found that this bug is actually a manifestation of two problems:
1. After grouping, _filter_bbox on the DrawingGroup representing the
layer is empty. It seems that SPGroup does not update the filter bbox
when a child is removed / added.
2. _drawbox on the same group is totally wrong (it has 1x1 size).
Problem 2 might actually be related to the wrongness of the
_filter_bbox code I mentioned earlier.