Does your game window looks like this?
 
  
Or like this?
 
  
In this guide I'll try to explain how to fix both of those issues so that the game can be displayed correctly.

First of all, it's recommended to have the game resolution set to either of those values. As visual bugs might occur when playing the game on resolutions higher than 1080p or lower than 720p.

You don't have to change your monitor display resolution.

If your game is zoomed in (see the first image in the guide), it's most likely because the Windows Scaling is set to a value higher than 100%. You can either set it to 100% and that'll solve the problem (I'll explain the Windows Scaling more in the next section).

There's also another solution if you don't want to change the Windows Scaling. This solution is to enable DPI scaling override.

To apply this change, first off you need to enter the directory containing the N++.exe file. Easiest way to do it is by choosing Properties of N++ on Steam, and from a new window select Local Files>Browse.

 
Select properties on the .exe file, then go to Compatibility>Change high DPI Settings.


 
From a new window, enable the checkbox Override high DPI scaling behaviour.
The value for Scaling Performed by should be set by default to Application



Close this window, apply changes in the Properties window, and that's all.

The solution for a zoomed out window (see the second image in this guide) isn't as simple. It usually means that a user has a monitor display higher than the in-game resolution. This issue is common for players with 1440p and 4k monitors.

The only solution I was able to find for this was also by using Windows Scaling. While in the previous section you had to decrease the Scaling value, this time you have to increase it so that the in-game resolution multiplied by the Scaling % matches your monitor resolution.

From Start Menu go to Settings>System>Display
In Scale and Layout change the value that's below to 200%.


 
For 200% scaling, use 1080p in game resolution if your monitor is 4k. For 1440p monitor, use 720p instead.

If a 720p game resolution isn't enough for you, then there's a way to play at 1080p, although the setup is even longer.

From Scale and Layout click on Advanced Scaling Settings.



Enter 133 as a scaling value. Then log out to apply changes.


 
You won't get exact 1440p resolution in game, but the difference will be too small to notice.

• If nothing happened after applying changes mentioned in the guide, try running N++.exe as an administrator.
  
• Make sure that High DPI scaling override is disabled when you're using the Windows Scaling solution