go to zk calendar demo, http://www.zkoss.org/zkdemo/application/zk_calendar
In Weekly mode, when dragging a Event to change it's time range(move mouse to bottom of a Event, when showing a resize-cursor, then drag the Event range),
sometime, it fail to drag. instead of, it shows a create-event dialog.

The forum discussion.
http://www.zkoss.org/forum/listComment/13706-Calendar-Event-Resize

The video.
http://www.youtube.com/watch?v=3Gotr0xsSy4