Apparently, this reproduces 100% of time:
FAIL: Timed out waiting for notifyDone to be called
Tests that shadow DOM tree is properly displayed.
- <meter id="meter" min="0" max="100" value="20">
<div style="width: 20%; "></div>
Before shadow root added
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests.
Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.