Selecting the source plan is pretty broken in Atlassian Bamboo version 5.12.0-m427 build 51209 - 04 May 16.

Tip: when you land in the view and select and item, it works straight away, but if you type something on the source plan input the component gets into an inconsistent state where mouse clicks are not getting handled properly.

Please have a look at the video.

You should be able to reproduce it easily in https://collaboration-bamboo.internal.atlassian.com/build/admin/edit/editBuildTasks.action?buildKey=FIL-HCME-DTA. Otherwise you can talk to me.