I want to ask pro developers out there that how they manage a big windows form class. Is it a good idea to split it with partial keyword across different files? That’s the thing that I was doing so far, but it creats unnecessry designer files that when you double click on them in VS, a blank winform will pop up:

So what I do, basically is group events and code logic for each related group of controls in one file.
My answer is “I don’t”. If you need a lot of code in one single class (in this case a form) it usually means your class is doing a lot of stuff and you need to make it less coupled. A good way to achieve this is to use a sort of MVC or MVP pattern form putting the logic in another place, and to use UserControls so you could have the different functionalities in different controls (with their controller or presenter, depending if you implement MVC or MVP). Divide and conquer.