Don't know what's the actual reason for such behavior apart from what I could see in log. ps. it happens in 4.5.0 & 4.5.1 18:24:02,155 DEBUG [HttpClientSupport$SoapUIHttpClient] Stale connecti...
I'm having the same "UI hangs" issue a few seconds after opening the REST Request Editor on a Mac. Tested with 4.5.1 and nightly build 4.5.1.1 from July 24.