Well, i have found one solution.
i have similar problem when i switch from laptop (1080p) to desktop (2K). It happens ramdomly, but more and more as i continue to work and update this same Vi. (I'm on LabVIEW 2014 x64)
there is black strips with scroll bars, and the front panel on the center is (probably) stayed at 1080p.
For the moment, the solution is to put (drag and drop) the top border of the Vi (front panel) to the border of the desktop (Windows). As expected, the front panel windows stretch from the top to the bottom of the screen.
And the strips disappear ! That's all