Apparently, this reproduces 100% of time: FAIL: Timed out waiting for notifyDone to be called Tests that shadow DOM tree is properly displayed. Running: testDumpInitial - <meter id="meter" min="0" max="100" value="20"> - (shadow) - <div> <div style="width: 20%; "></div> </div> </meter> Running: testDumpUpdated Before shadow root added <div id="delayed-shadow-host"></div>
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.