[Trunk] Cannot drag between staves

1. Create piano score.
2. Using your mouse, hold-click between the staves and move the mouse around.

Expected result: The page moves.
Actual result: Only the mouse moves (cursor shows no change either).

(Workaround: Choose somewhere outside the staves)

I noticed this issue when I attempted to drag-select.

Using MuseScore 2.0 nightly build (4156) - Mac 10.4.11.