why mouse event handler in ui.core.js will return false default?

why mouse event handler in ui.core.js will return false default?


hi Dears,
Recent days I tried to put a combobox in a draggable layer(applied the
draggable plugin) but I encountered a problem that the combobox list
can't hide when I mouse down on layer.
and finally, I found the root cause is the mouse events (e.g.
mouseDown, mouseMove, mouseUp) in the ui.core.js will always return
false and then jQuery will stop to bubbling the mouse down event. And
actually, my combobox will hide when mouse down event occur within
document.
So, I'd like to know is there any reason that jQuery UI team expect
those mouse events should always return false? or any clues I can
refer to let my combobox working together with draggable plugin?