In Preferences->Connections tab, proxy server address and port always appear enabled

On the Connections tab of the Preferences dialog, the Proxy Server section has a checkbox to enable use of a proxy server, and two textboxes for the address and port of the proxy server. When the proxy server checkbox is not ticked, the textboxes cannot be edited but appear enabled - they should be greyed out for consistency, and appear enabled when the proxy server checkbox is ticked.