### 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
void RefreshTouchBarItem(const std::string& item_id) override;
  void SetEscapeTouchBarItem(gin_helper::PersistentDictionary item) override;
  void SelectPreviousTab() override;
  void SelectNextTab() override;
  void MergeAllWindows() override;
  void MoveTabToNewWindow() override;
  void ToggleTabBar() override;
  bool AddTabbedWindow(NativeWindow* window) override;
  void SetAspectRatio(double aspect_ratio,
                      const gfx::Size& extra_size) override;
  void PreviewFile(const std::string& path,
                   const std::string& display_name) override;
  void CloseFilePreview() override;
  gfx::Rect ContentBoundsToWindowBounds(const gfx::Rect& bounds) const override;
  gfx::Rect WindowBoundsToContentBounds(const gfx::Rect& bounds) const override;
  void NotifyWindowEnterFullScreen() override;
  void NotifyWindowLeaveFullScreen() override;
  void NotifyWindowWillEnterFullScreen();
  void NotifyWindowWillLeaveFullScreen();
  
  
  
  void Cleanup();
  
  void OverrideNSWindowContentView();
  
  void SetStyleMask(bool on, NSUInteger flag);
  void SetCollectionBehavior(bool on, NSUInteger flag);
  void SetWindowLevel(int level);
  enum class FullScreenTransitionState { ENTERING, EXITING, NONE };