To avoid undesirable stretching when using Unity in orthographic mode, make sure that the camera's aspect ratio matches the resolution of the game window.
- Go to Edit -> Project Settings -> Player.
- In the Resolution and Presentation section, set the aspect ratio to be the same as your game window's aspect ratio.
- Make sure that your game window's aspect ratio is the same as your camera's aspect ratio. You can set the camera's aspect ratio in the Camera component's settings.
- Make sure that your sprites and UI elements are all displayed at their native resolution. You can do this by setting the "Pixels Per Unit" value of each sprite to match its pixel dimensions.
By ensuring that all aspects of your game are set to their native resolution and aspect ratio, you can avoid unwanted stretching in orthographic mode.