User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/11.1.2 Safari/605.1.15 Steps to reproduce: 1. Open developer tools 2. Enter Responsive Design Mode 3. Refresh page Actual results: The column that typically displays the styles of the selected element in the inspector is blank with a message "No element selected", and remains blank regardless of what element actually is selected. This panel remains blank until Dev Tools is closed and reopened, or a new window/tab is created. Expected results: The styles of the selected element should be displayed with checkboxes to toggle etc. This has been present in the Nightly build for at least a week.