I'm using a 16:9 display and watching 4:3 content, why is the top/bottom of the screen cropped?

By default Zoom Player is designed for a 4:3 display which means that any image sizing is done on the vertical axis, which causes the image to be cropped on a 16:9 display. To fix this, open the Advanced Options Dialog and under "Playback / Video / Aspect Ratio", make sure that "Restrict AR Adjustments to Height in Fullscreen / Zoom mode" is NOT checked.