#776 Performance peak when moving mouse over Event Sheet

By simply moving the mouse over the Event Sheet Editor, the processor usage is vastly increased.
On a 1.8 ghz dual-core, wiggling the mouse over an empty sheet pushes processor usage to 60% for a single core.
A medium-sized event sheet can max processor usage of a single core to 100%.

Ashley suspects that the Event Sheet is (unnecessarily) redrawn whenever the mouse has moved (http://www.scirra.com/phpBB3/viewtopic.php?f=2&t=3230).