button-press-event causes leave-notify-event and enter-notify-event to be called in rapid succession.
This means, when changing fill-color on enter-notify-event and changing it back to it's original color on leave-notify-event (to simulate a "selected" item), you get a "flashing" background whenever you click that item in question.
An example of this can be seen in the arrowhead demo with the resize boxes. It gets lot's worse with larger items.
Note button-release-event does not have this problem.