Offset field not editable after resetting to default when multiple elements are selected

Steps:

• Create a new piano score.
• Click on the rest in the first measure of the treble clef. In the Inspector, change its vertical offset for "0.50sp".
• Click on the rest in the first measure of the bass clef. In the Inspector, change its vertical offset for "1.00sp".
• Now, select both rests. In the Inspector, reset the vertical offset to default by clicking the button with arrows (button to the right of the line "Vertical offset"). The field is changed to "0.00sp".
• Try to change the vertical offset by entering numbers in the field. Result: the field can't be edited, as if it was read only.
• Try to change the vertical offset by clicking the up or down arrows. Result: the two rests move up or down, but the field keeps displaying "0.00sp".

Note: the bug only occurs if the elements selected don't have the same offset. If we try the steps above with two elements having the same vertical offset (for example both to "1.00sp"), the bug doesn't occur.

Config: MuseScore f5b9240 on Xubuntu 14.10.