Hi
In the earlier version of VP, using the Crop option and selecting to "Force" 16:9 resulted in the image being cropped to 16:9 AND being enlarged to fill the screen. No matter which side you pulled in the ratio remained 16:9 and the outlined area filled the screen
In version 3.55 (and presumably 3.57) selecting Crop and forcing 16:9 followed by altering the display rectange simply reduced the frame in size down to the rectangle leaving a black border. (The cropped area remained 16:9 though. I initially thought this was a bug until Borate pointed out that I had to tick the "Zoom to the cropped area" box to make it behave like the earlier version.
Is this what you are seeing?
In 3.55 there is a slight grey border at the top that seems to be connected with the preview screen title lettering, but in practice this doesn't cause a problem.
Nat