Description of problem: https://github.com/stransky/gecko-dev.git package crated when clipboard contains some data before browser start and user pastes to browser after browser start. Looks like data_offer_offer() callback collides with the one in Gtk and we don't get data_device_selection() callback when browser is focused. Seems to a race condition between our and Gtk clipboard handling.
Fixed by commit 74cb6f1445ed2c79d746bebf2703ebb45a8d2008