User agent selection in F12 should be improved
The F12 tools should improve how the user-agent override is exposed in two ways.
For Custom, the edit control is too tiny. It should be a large, even multi-line control with wrap. The existing control is too small to see any reasonable user-agent.
For all other selections, F12 doesn't display the value used. My suggestion is to all use the same multi-line control suggested above but that the control is read-only when any value other than custom is selected. Make sure that it is read-only but not disabled so that users can select and copy the value that will be used.