Comment 18 for bug 95619

Turns out that this was because of a rounding error in the communication between ubiquity and partman, which led ubiquity to believe that the current size of the partition was outside the resize bounds, which then caused the spinbutton to behave oddly. Really the rounding error ought to be fixed, but that involves some harder work down in the plumbing. In the meantime, I've just worked around it in the frontends by having them extend the bounds of the spinbutton as necessary.

The original problem #1 still remains, but should be much less severe now.