I have no idea why it's happening, maybe file a bug report.
Adding vertical padding of 1 fixes it.
struct ContentView: View {
@State var myText = "Hello, World"
var body: some View {
VStack {
TextEditor(text: $myText)
.frame(height: 200)
.border(Color.red)
Spacer()
}
.padding(.vertical, 1)
}
}