wlroots 0.3 contains the following breaking changes:

• gtk-primary-selection: refactor everything, untie from seat (#1397). Various properties related to this unstable protocol have been removed from wlr_seat and wlr_seat_client. Use wlr_gtk_primary_selection_* instead.
  
• output: switch wlr_output_swap_buffers damage to output-buffer-local coords (#1403)
  
• Standardize the wlr_box input paramaters (#1441). The destination parameter is now always the first one.
  
• Remove wlr_xdg_surface_send_close (#1461). Use wlr_xdg_toplevel_send_close and wlr_xdg_popup_destroy instead.
  
• data-device, primary-selection: add request_set_selection (#1402). Compositors now need to listen on wlr_seat.events.request_set_selection and wlr_seat.events.request_set_primary_selection, then decide whether or not to accept the request with wlr_seat_set_selection and wlr_seat_set_primary_selection.
  
• wlr_seat_client_send_selection has been removed, this is an internal function (#1427)
  
• wlr_data_source_finish has been removed, this is now handled by wlr_data_source_destroy (#1427)
  
• wlr_data_source_cancel has been renamed to wlr_data_source_destroy (#1427)
  
• Compositors now need to listen on the wlr_pointer.frame or wlr_cursor.frame events, and call wlr_seat_notify_frame (#1503). If you're simulating pointer events, you'll need to call wlr_seat_notify_frame manually.