### Preflight Checklist

- [X] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
- [X] I have searched the [issue tracker](https://www.github.com/electron/electron/issues) for a feature request that matches the one I want to file, without success.

### Electron Version

13.0.0-beta.20

### What operating system are you using?

macOS

### Operating System Version

11.3.1

### What arch are you using?

arm64 (including Apple Silicon)

### Last Known Working Electron version

_No response_

### Expected Behavior

Naturally, the window's borders should be rounded, with nothing obscure protruding amongst them--And they should stay that way _after_ maximization, like so:
![Screen Shot 2021-05-07 at 2 48 02 PM](https://user-images.githubusercontent.com/83843298/117511979-8d1ba180-af43-11eb-99f6-5163877619f4.png)


### Actual Behavior

But, for some weird reason, after (and while) the window is maximized, weird looking white corners appear on every point of the window's border radius...
![Screen Shot 2021-05-07 at 2 49 47 PM](https://user-images.githubusercontent.com/83843298/117512201-074c2600-af44-11eb-9f20-3017207cbea5.png)


### Testcase Gist URL

https://gist.github.com/TurnipTotSoftware/74db97bee9b29183006fd1204cfff746

### Additional Information

This glitch also happens after my Mac sleeps for a bit...
Hopefully this problem is an easy fix!

Good day👍
shell/browser/native_window_mac.h
FullScreenTransitionState::NONE;
  NSInteger attention_request_id_ = 0;  
  
  NSApplicationPresentationOptions kiosk_options_;
  
  TitleBarStyle title_bar_style_ = TitleBarStyle::kNormal;
  
  VisualEffectState visual_effect_state_ = VisualEffectState::kFollowWindow;
  
  
  base::Optional<bool> window_button_visibility_;
  
  bool maximizable_ = true;
  
  bool always_simple_fullscreen_ = false;
  bool is_simple_fullscreen_ = false;
  bool was_maximizable_ = false;
  bool was_movable_ = false;
  NSRect original_frame_;
  NSInteger original_level_;
  NSUInteger simple_fullscreen_mask_;
  base::scoped_nsobject<NSColor> background_color_before_vibrancy_;
  bool transparency_before_vibrancy_ = false;
  
  NSApplicationPresentationOptions simple_fullscreen_options_;
  DISALLOW_COPY_AND_ASSIGN(NativeWindowMac);
};
}  
#endif