Chrome app: sandbox resource address unreachable

I am trying to set a webview's source using a local file.

Manifest:

  "webview": {
    "partitions": [{
      "name": "static",
      "accessible_resources": [
        "sandbox/sandbox.html",
        "sandbox/sandbox.js",
        "sandbox/sandbox.css",
      ]
    }]
  },

Set Webview's source

  var webviewSrc = chrome.runtime.getURL('sandbox/sandbox.html');
  //console.log prints chrome-extension://MY_CHROME_APP_ID/sandbox/sandbox.html
  webviewEl = <webview id="webview" ref="Webview" src={webviewSrc} partition="static"></webview>

Error:

<webview>: The load has aborted with error -109: ERR_ADDRESS_UNREACHABLE.

Why is it happening, and how do I fix this issue?

This setting works:

/foreground/main.html

<html>
  <head>
    <link rel="stylesheet" type="text/css" href="./bundle/app.css">
    <link rel="stylesheet" type="text/css" href="./main.css">
  </head>
  <body>
    <div id="app"></div>

    <div id="webview-container">
      <webview id="youtube-webview" src="sandbox/sandbox.html" partition="static"></webview>
    </div>

    <script src="./bundle/app.js"></script>
  </body>
</html>

/sandbox/sandbox.html

Hello

This setting returns 'address unreachable' error:

/foreground/main.html

<html>
  <head>
    <link rel="stylesheet" type="text/css" href="./bundle/app.css">
    <link rel="stylesheet" type="text/css" href="./main.css">
  </head>
  <body>
    <div id="app"></div>
    <script src="./bundle/app.js"></script> <-- initialize webview inside here
  </body>
</html>

/sandbox/sandbox.html

Hello

/foreground/bundle/app.js

var webviewSrc = chrome.runtime.getURL('sandbox/sandbox.html');
// or var webviewSrc = 'sandbox/sandbox.html';
webviewEl = _react2.default.createElement("webview", { id: "webview", ref: "Webview", src: webviewSrc, partition: "static" });
// ADDRESS_UNREACHABLE

<webview> is for external content.

To embed your own sandboxed pages, use <iframe>.

Also, apparently it's important to use relative paths - getURL returns a different URI scheme from what's expected.