When I do g_timeout_add_seconds(1, (GSourceFunc)timer_timeout, t); in glib 2.26, it fires once per second, with 2.28 it fires every second second. If I pass 0, it fires very often in 2.26, every second in 2.28, and so on.

The problem is located in the g_timeout_set_expiration() function I think. I tried comparing it with the 2.26 code, but I couldn't figure anything out. The only way I could fix it was to replace the whole logic with
  timeout_source->expiration += timer_perturb - (timeout_source->expiration % 1000000);