Play in editor, New Editor Window squeezed down by my PC-screen resolution

Im making my first ever mobile phone game.
The game is made in the resolution 720x1280, which then will scale up to larger resolutions depending on the phone.

In Editor Preferences > Level Editor > Play > Game Viewport Settings > New Viewport Resolution, i have it set to 720x1280.

When i play in the New Editor Window that 720x1280 gets squeezed down to 720x1080 so that it fits my 1920x1080 pc screen. This is a problem because things like UI elements are not in the right places.

How do i make it so the the New Editor Window size actually is 720x1280 even if it goes outside my pc screen?