The application currently is not prepared to correctly handle any other disk label type than msdos. Until that situation changes, all disk labels but msdos are "read only".
Don't assume we always want to snap to the front. Always trying to snap to the front solves the problem that a partition does get too small to take another one that's copied to it, but it introduces a new bug: The user might create a partition aligned at the end of a device, extended partition or at the start of the next one, but we snap to the back and leave some space in between.
The ResizeDialog does not yet have a "dirty bit". It should get one and disable its OK button as long as nothing has been modified, much like the PartPropsDialog already does.
if this fails, no one undoes the shrinking of the file system above, because we rely upon there being a maximize job at the end, but that's no longer the case.
Wouldn't it be better to have newfirst (new first sector) and newlast (new last sector) as args instead? Having a length here doesn't seem to be very consistent with the rest of the app, right?