The right border and lower border of Button is incorrect when setting button's FlatStyle as Flat or Popup.

[Amy-Li03]

• .NET Core Version:
  5.0.100-rc.1.20426.12

• Have you experienced this same bug with .NET Framework?:
  No

• More info:

1. This is a regression issue.
2. It cannot reproduce on .NET core 3.1 & .NET 5.0 Preview8.

Problem description:
When set button's FlatStyle as Flat or Popup, the right border line of button is not overlap.(lower border also exist this issue)

Expected behavior:
The right border and lower border of button should display as following video.

Minimal repro: TestApp.zip

1. Create a new winforms .NET core project and target to .NET 5.0.
2. Add a Button to the form designer.
3. Set the FlatStyle of Button as Flat.
4. Build and run the project.
5. Observe the right border and lower border of button.

[Amy-Li03]

Verified this issue on .NET 5.0 RC2 latest build 5.0.100-rc.2.20459.24, issue was fixed. The right border and lower border of Button display correctly when setting button's FlatStyle as Flat or Popup.