Minecraft's window width defaults to 854 when launching from the latest launcher

When I started up minecraft yesterday after a new launcher update. I noticed that the window side was thinner than normal (I run my game at 1600x900) so I took a screenshot of the main menu and saw that the resolution was 854x900. So I resized the screen to 800x600 and I knoticed that the width of the window was wider, so I looked at another screenshot of it and it was yet again 854x600.