Windows key + arrow, i.e. Aero Snap, performing differently in Windows 10
Solution 1:
You can edit this behavior. Windows 10 has 'MultiTasking' options in Settings which will fix your problem. In MultiTasking, turn off the setting 'When I snap a window, show what I can snap next to it".
When you turn this setting off, the window retains the focus (as it should by default in my opinion)