When using Small interface size, in the Effects section of the Trigger dialog, the pop-up menus having lots of items (e.g. Music) require down-arrow keys followed by a click on the pop-up menu list selector (small down arrow icon) to select an item. Items cannot be selected with mouse click. With other interface sizes (I only tested Very Small and Normal), the issue does not occur and items in drop-down lists can be selected with mouse click.
True in 1.66 stable branch and 1.68 RC.