Whenever row changes or which row is before row changes this is called, which lets you update the header on row. You may remove or set a new one via the gtk-list-box-row-set-header-func function or just change the state of the current header widget.

 lambda (row before)  

row

A gtk-list-box-row widget with the row to update.

before

A gtk-list-box-row widget before row, or nil if it is the first row.