[trunk] R5368 Thumbnails in nav window not wide enough

Create a score with 1 page. In the nav window drag the blue box all the way to the top-right. The thumbnail has a small amount of the page cut off the right-side (a few mm) as you can still drag the score a small amount to the left before the page margin appears. Now add a second page and the amount of image missing from the right side of the last thumbnail is slightly larger again. This narrowing of the final thumbnail will continue as more pages are added. See attached pic with a 4 page score. The final page is cut off into the score.

Win7 32-bit