ui.List

The default functionality for ui.List is to select an item on mouseup.

How would you change it so that mousedown selects an item?

You can modify the keyboard/mouse controller to use onItemMouseDown event instead of onItemClicked ( source/lib/ui/_single.js ) or just add the event on top of the existing list -

list.onItemClicked = function(){return 1}; // cancel existing handler
list.onItemMouseDown = function(event, i){
this.raiseEvent("selectClickedItem", event, i);
};