Right click on lower staff selects top staff (0.9.6 regression)

Steps to reproduce bug
1. Open a multi-staff score such as bach-bc2 demo
2. Right click on the empty part of a measure in a lower staff (any instrument other than the top one).

Expected behavior: Blue box normally appears around the measure you just clicked on.

Actual behavior: A blue box appears around a measure in the top staff instead of the one you clicked on

MuseScore version: r. 2572 nightly

(Operating System: Windows XP)