Cannot adjust endpoints of spanners between systems

Steps to reproduce:
1) Create a new score with at least two systems
2) Place notes that span the two systems
3) Select a note in the first system and press S to create a slur. Shift-Right arrow until it extends into the following system.
Expected: The operation should be undoable (i.e., Shift-Left arrow should bring the slur back into the first system.)
Actual: As soon as the slur is broken into two systems, the right endpoint of the slur can no longer be adjusted, and Ctrl-Z does not restore the unbroken slur.

When the slur is broken, the first half's starting point can be adjusted. The second half's ending point cannot be adjusted. There is a weird case when you attempt to adjust the first half's ending point (which is the end of the system) and the second half's starting point (the start of the following system); perhaps the user should be prevented from performing such actions (visual adjustment only).

This appears to be the behaviour of other spanners, too, like hairpins.

R.7e8e83b Nightly Build
Operating system: Windows 8

GIT commit: 7e8e83b