title: Excel Add-In running on Windows "fails to fetch" when attempting to communicate with a local https process
body: <p>I want to preface this issue by stating that we have successfully gotten our add-in to work with this communication scheme on Mac.</p>
  <p>What we are doing is that we have a local HTTPS process (running on the same host that Excel is running on), and we are attempting to communicate with said process utilizing the fetch API.</p>
  <pre><code>+-------------+     +-------+--TASKPANE--+
  | lightweight |     | Excel |            |
  | HTTPS       |&lt;---------------- fetch() |
  | process     |     |       |            |
  +-------------+     +-------+------------+
  </code></pre>
  <p>As you may have guessed, we jump through a number of hoops in order to make the request work:</p>
  <ul>
  <li>Our process serves up the necessary CORS-headers</li>
  <li>The embedded HTTP server uses a self-signed certificate, but the custom CA is installed into the system's root trust-store (both on Mac/Windows)</li>
  <li>An entry is added to <code>/etc/hosts</code> (for Mac) and <code>C:\system32\windows\drivers\etc\hosts</code> (for Windows) that resolves <code>desktop.ourdomain.io</code> =&gt; <code>127.0.0.1</code></li>
  </ul>
  <p>On Mac, this is enough for the add-in to be able to communicate with the lightweight process over HTTPS without any complaint.  However, on Windows the embedded browser fails with the message &quot;failed to fetch&quot;.  It is worth noting that this communication topology <em>does</em> work, however, in a browser on Windows, just <strong>not</strong> in the Task Pane.</p>
  <p>As a last resort, I added <code>desktop.ourdomain.io</code> to the domain list in the Office Add-In manifest, and then tried to browse to a known-good endpoint at that domain from within the Task Pane.  It said something like &quot;Excel could not connect with the add-in&quot; (I forget the exact error), and showed a &quot;Retry&quot; button, as if it was ignoring the hosts entry.</p>
  <p>Perhaps somebody from the Excel/Windows team could weigh in on how we can open up communication between the Task Pane and our process?</p>