This means that you can't do any work in Geany until you close the external diff viewer. This seriously reduces the usefulness of the otherwise great feature of being able to use an external diff viewer.
The external diff viewer is spawned here: https://github.com/geany/geany-plugins/blob/master/geanyvc/src/externdiff.c#...
Looks like the problem might simply be solved by calling `g_spawn_async` instead of `g_spawn_sync`.