ID7 includes a reflow plugin which fires id7:reflow events when the browser is resized in such a way as to change the current breakpoint (e.g. from lg to md). Event data is an instance of the ReflowEvent class.
We only expect the event to fire when the size of the main container changes. In other words, you'll receive one event on the transitions from [lg to md, md to sm, sm to xs] and an event every time the browser notifies us of a size change for the xs size.
Sizes are defined in the aptly-named screen-sizes.js source file.
This page shows a message whenever the event is fired. Try resizing your browser: