You can inspect and modify the order of widgets on a card via their .index attribute, which counts from back to front, starting at 0. This is automatically clamped within the permissible range on a card, so assigning it to 0 or -1 can be a convenient shorthand for "all the way to the back" and any very large number (say, 999999) can likewise be used for "all the way to the front".
For example, you could give your draggable canvas a script like so:
on click do me.index:999999 end