The white border around the Image View Area is the Sensor crop region. This shows where the image will be cropped after capture in viewing applications (like the Lytro Desktop 4 Software).
Portions of the picture that are outside of the sensor crop region will only show when the perspective is changed in a viewing application (Lytro Desktop 4, or in Lytro's Web Gallery, for example).
Why does this happen? The answer is this: Think about how you see perspective in real life - hold your hand a few inches in front of one eye, and although you can see "around" your hand using information from your other eye, part of the depth information is missing for the area covered by your hand. This is the same at the margins of a living picture as you move around in Perspective Shift. Rather than have unappealing black margins, or missing data, we allow enough room on the edges of the image to allow for enough data to be captured to fill in all the details.