Called when drag will be applied. If dragEnd was called with apply false, it will not be called. Return false to not apply the regular drag end changes (i.e. return false to reset to the position before dragging).
Called when the drag coordinates change (during any event). Should return true to allow the edges to be moved, or false to prevent it.
Can be used to save some context/info to later apply safely during onDragApply.
Whether to show merged the moved frames while dragging.