Booting my Thinkpad X200 from an Ubuntu 16.04 live USB key, GParted was able to successfully shrink a NTFS partition and leave unallocated space. Then, a request to either (i) shrink the existing \ EXT4 partition, or (ii) create a new empty Home EXT4 partition in the unallocated space resulted in a message:
Unable to satisfy all constraints on the partition
Following the trail of many others with the same issue, I tried Testdisk, which analyzed the partition structure. The structure was found as expected. Specifying the “primary bootable”, “primary”, and “logical” partitions seemed okay, but a “Bad Structure” warning would not be resolved unless the “Lenovo Recovery” partition was marked for deletion. This caused me to pause, as that’s the way that Windows 7 could be reinstalled to the factory state (albeit on a larger disk than the original hardware build).
As I’ve been moving over from Ubuntu 16.04 to Kubuntu 17.04, there was nothing to be lost by trying instead with a Kubuntu live USB key. Using KDE Partition Manager, the repartitioning went smoothly, without the error message from GParted that I saw over and over, for hours.
Conclusion: There’s more to Kubuntu than just a change in the Desktop Environment.
All of this effort was towards migrating the home directory into its own dedicated partition.