Why does the viewport become like this? Before hitting play everything is normal and when I click Play, it turns like this .
You can always test your game in New Editor Window with your desired size, just click the arrow near the play button and select “Advanced Settings”:
Then, go to Game Viewport Settings and set the desired window size:
Finally, go back to Play Modes and select the New Editor Window:
After this, whenever you hit play, the new game screen with your set size will open.
Hope it helps!