### 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
uint32_t changed_metrics) override;
 private:
  
  void AddContentViewLayers();
  void InternalSetWindowButtonVisibility(bool visible);
  void InternalSetStandardButtonsVisibility(bool visible);
  void InternalSetParentWindow(NativeWindow* parent, bool attach);
  void SetForwardMouseMessages(bool forward);
  ElectronNSWindow* window_;  
  base::scoped_nsobject<ElectronNSWindowDelegate> window_delegate_;
  base::scoped_nsobject<ElectronPreviewItem> preview_item_;
  base::scoped_nsobject<ElectronTouchBar> touch_bar_;
  base::scoped_nsobject<WindowButtonsView> buttons_view_;
  
  id wheel_event_monitor_;
  
  
  
  base::scoped_nsobject<NSView> container_view_;
  
  std::unique_ptr<RootViewMac> root_view_;
  bool is_kiosk_ = false;
  bool zoom_to_page_width_ = false;
  bool resizable_ = true;
  base::Optional<gfx::Point> traffic_light_position_;
  std::queue<bool> pending_transitions_;
  FullScreenTransitionState fullscreen_transition_state() const {
    return fullscreen_transition_state_;
  }
  FullScreenTransitionState fullscreen_transition_state_ =