I do not see that keybind in any of the config files or in the in game config.

Originally posted by @rhit-lindsekb in #773 (comment)

Using this bind in the tag screen will toggle

@emilyploszaj I do not want to set an item to a default, i want to be able to remove it from being the default item.

"Default Stack" is what you're looking for, ctrl left click by default