WinControl

In the eventuality that showing windows side by side in Windows 10 does not work for you, then you should consider modifying the Settings. Generally speaking, the problem occurs because the option 'When I snap a window, show what I can snap next to' from Snap, Multitask is not checked. Make sure it is checked and after you reboot your computer, the problem should be fixed.

If the issue persists and you need to have windows side by side for your school or work projects, then you can consider third party solutions. WinControl is a tiny tool designed specifically for the side by side and window stacked commands not working properly.

The installation is a simple and straightforward process that does not require any special attention on your part. The program comes with a small interface and automatically detects the windows with full screen capabilities. The app immediately adds and removes windows that fit this description, but there is a chance that other programs opened can also be detected and displayed in the list.

In the lower bar of the UI, you can find out the exact position of the mouse, namely on the X and Y coordinates. You can navigate through the app shown there using the Up and Down buttons. As you probably hinted, the Split and Stack buttons allow you to display all windows side by side, thus sparing you the trouble of arranging them individually and not having to worry about the empty space.

On a side note, even though there is a Kill button there as well, it is not active or at least it is not clear whether this is not available during the trial mode.

In case everything you tried failed to address this frustrating matter and having windows side by side is important for your needs and preferences, then perhaps you can consider giving WinControl a try.