<h3 dir="auto">Preflight Checklist</h3>

<ul class="contains-task-list">
<li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox" checked=""> I have read the <a href="https://github.com/electron/electron/blob/master/CONTRIBUTING.md">Contributing Guidelines</a> for this project.</li>
<li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox" checked=""> I agree to follow the <a href="https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md">Code of Conduct</a> that this project adheres to.</li>
<li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox" checked=""> I have searched the issue tracker for an issue that matches the one I want to file, without success.</li>
</ul>
<h3 dir="auto">Issue Details</h3>
<ul dir="auto">
<li><strong>Electron Version:</strong> v12.0.0-beta.28</li>
<li><strong>Operating System:</strong> macOS 11.1</li>
</ul>
<h3 dir="auto">Actual Behavior</h3>
<p dir="auto"><a target="_blank" rel="noopener noreferrer nofollow" href="https://user-images.githubusercontent.com/1093313/108893624-50742580-7637-11eb-8aed-43f522583a81.png"><img width="713" alt="Screenshot 2021-02-24 at 12 29 08 AM" src="https://user-images.githubusercontent.com/1093313/108893624-50742580-7637-11eb-8aed-43f522583a81.png" style="max-width: 100%;"></a></p>
<p dir="auto"><a target="_blank" rel="noopener noreferrer nofollow" href="https://user-images.githubusercontent.com/1093313/108894344-2b33e700-7638-11eb-9d2d-b7f81b76b02c.png"><img width="80" alt="Screenshot 2021-02-24 at 12 35 13 AM" src="https://user-images.githubusercontent.com/1093313/108894344-2b33e700-7638-11eb-9d2d-b7f81b76b02c.png" style="max-width: 100%;"></a></p>
<p dir="auto">The borders are incorrectly drawn. It was fine in macOS Catalina.</p>
<h3 dir="auto">To Reproduce</h3>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="  const mainWindow = new BrowserWindow({
    width: 800,
    height: 600,
    titleBarStyle: 'hiddenInset',
    vibrancy: 'sidebar'
  })"><pre class="notranslate"><code class="notranslate">  const mainWindow = new BrowserWindow({
    width: 800,
    height: 600,
    titleBarStyle: 'hiddenInset',
    vibrancy: 'sidebar'
  })
</code></pre></div>
<p dir="auto">Fiddle Gist: <a href="https://gist.github.com/0d9f9327689a795cc95f1c046cbed488">https://gist.github.com/0d9f9327689a795cc95f1c046cbed488</a></p>
<h3 dir="auto">Preflight Checklist</h3>

<ul class="contains-task-list">
<li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox" checked=""> I have read the <a href="https://github.com/electron/electron/blob/master/CONTRIBUTING.md">Contributing Guidelines</a> for this project.</li>
<li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox" checked=""> I agree to follow the <a href="https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md">Code of Conduct</a> that this project adheres to.</li>
<li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox" checked=""> I have searched the issue tracker for an issue that matches the one I want to file, without success.</li>
</ul>
<h3 dir="auto">Issue Details</h3>
<ul dir="auto">
<li><strong>Electron Version:</strong>
<ul dir="auto">
<li>11x</li>
</ul>
</li>
<li><strong>Operating System:</strong>
<ul dir="auto">
<li>macOS 11.1 [Big Sur]</li>
</ul>
</li>
<li><strong>Last Known Working Electron version:</strong>
<ul dir="auto">
<li>

</li>
</ul>
</li>
</ul>
<h3 dir="auto">Expected Behavior</h3>
<p dir="auto">when using BrowserWindow vibrancy effects eg: <code class="notranslate">vibrancy: 'sidebar'</code>, the corners of the window are squared.</p>
<h3 dir="auto">Actual Behavior</h3>
<h3 dir="auto">To Reproduce</h3>
<p dir="auto">Create a new BrowserWindow with vibrancy effects eg: <code class="notranslate">vibrancy: 'sidebar'</code>.</p>
<p dir="auto"><a href="https://gist.github.com/262108f4d281d9aa780ecc18f7ade2ba">https://gist.github.com/262108f4d281d9aa780ecc18f7ade2ba</a></p>



<h3 dir="auto">Screenshots</h3>
<p dir="auto"><a target="_blank" rel="noopener noreferrer nofollow" href="https://user-images.githubusercontent.com/1267580/102683285-fcc90400-41cf-11eb-84af-e379d4aba23b.png"><img width="454" alt="squared-corners" src="https://user-images.githubusercontent.com/1267580/102683285-fcc90400-41cf-11eb-84af-e379d4aba23b.png" style="max-width: 100%;"></a></p>
<h3 dir="auto">Additional Information</h3>
<p dir="auto">Adding <code class="notranslate">titleBarStyle: 'hidden'</code> rounds the corners, but the corner radius is smaller that normal windows in Big Sur.<br>
<a target="_blank" rel="noopener noreferrer nofollow" href="https://user-images.githubusercontent.com/1267580/102683360-b6c07000-41d0-11eb-9dac-59d882f38422.png"><img width="114" alt="small-radius" src="https://user-images.githubusercontent.com/1267580/102683360-b6c07000-41d0-11eb-9dac-59d882f38422.png" style="max-width: 100%;"></a><br>
Which seems to be the issue shown here: <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="769507676" data-permission-text="Title is private" data-url="https://github.com/electron/electron/issues/27061" data-hovercard-type="issue" data-hovercard-url="/electron/electron/issues/27061/hovercard" href="https://github.com/electron/electron/issues/27061">#27061</a></p>