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### Issue Details

* **Electron Version:**
  * 6.0.0-beta.15
* **Operating System:**
  * Windows 10 (1903)
* **Last Known Working Electron version:**
  * 5.0.8

### Expected Behaviour
When a frameless window is maximized, and then enters full screen from that state, it should fill the entire screen with no border.

### Actual Behaviour
If a frameless window enters fullscreen from a maximized state, a ~10px white border appears all around the screen.
(Hard to show in a screenshot on a white page…)

### To Reproduce
Fiddle gist: https://gist.github.com/caesar/12c97746e9d8b0e9bd2ee7e4958c63ad

### Screenshots
It's hard to see the issue here because the border is white and so is the background of this page… but if you look carefully, the second time I the window enters fullscreen (when it does it from maximized), a white border appears.
![Kapture 2019-07-26 at 23 28 21](https://user-images.githubusercontent.com/1566516/61984625-20991200-affd-11e9-9e8a-9b4faed1fea8.gif)

shell/browser/native_window_views_win.cc
WPARAM w_param,
                                                  LPARAM l_param) {
  if (n_code < 0) {
    return CallNextHookEx(NULL, n_code, w_param, l_param);
  }
  
  
  
  if (w_param == WM_MOUSEMOVE) {
    for (auto* window : forwarding_windows_) {
      
      
      
      
      RECT client_rect;
      GetClientRect(window->legacy_window_, &client_rect);
      POINT p = reinterpret_cast<MSLLHOOKSTRUCT*>(l_param)->pt;
      ScreenToClient(window->legacy_window_, &p);
      if (PtInRect(&client_rect, p)) {
        WPARAM w = 0;  
        LPARAM l = MAKELPARAM(p.x, p.y);
        PostMessage(window->legacy_window_, WM_MOUSEMOVE, w, l);
      }
    }
  }
  return CallNextHookEx(NULL, n_code, w_param, l_param);
}
}