How to set aspect ratio?
1. Press on the screen while the video viewing mode to show the app's toolbar.
2. Press the button Settings 
in the lower left corner.
3. Choose
a. Content mode→Fill width to fit the video by its width.
b. Content mode→Fill height to fit the video by its height.
c. Content mode→Zoom to fit the video by proportional zoom.
d. Aspect ratio→Original / 16:9 / 4:3 to choose screen aspect ratio.
