Isn't windows forms designer supported more?
Answered
I saw this message after update.

Should I change to .NET from .NET Core to use Rider designer?
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Hello Akinori,
Winforms designer was not supported previously for .Net Core, here is the corresponding issue: RIDER-58357, we would appreciate it if you would upvote in order to bring increased awareness to the issue.
I could use winforms designer at previous version so I thought it was supported.
Thank you for information!