When running GTK3 apps such as gedit or virt-manager over a remote SSH connection using X forwarding, the remote app sometimes stops rendering for long periods of time (30 seconds or more) or forever.

I can easily reproduce this on my machine (Fedora 21, Intel Haswell integrated graphics, fully updated) by starting up a F21 virtual machine in KVM and SSHing into the guest from the host machine. After starting up gedit in the remote session and typing a few characters, the gedit window stops updating and the menu buttons, etc. stop responding properly. Sometimes if you leave it that way long enough it will update eventually, but then won't update again for a long time.

The strange thing is that it seems to be timing-dependent. In one setup, I can log into two remote systems (CentOS 7 in this case) with the same package versions, and one has issues and one doesn't. The one that works is the slower machine, the faster machine has issues. So my guess is that whether the problem shows up depends on how fast the machine being logged into is (and likely the speed of the network connection). The case where you log into a VM running on the local machine seems to reproduce it quite reliably for me however.

As far as I can tell, only GTK3 apps seem to be affected. For example, gedit and virt-manager on F21. gvim (vim-X11) appears to be fine however, and uses GTK2.

The other piece of information that may or may not be important is that both of the systems I've seen this happen on (i.e. the systems logging into the other machine remotely) were running Intel Haswell graphics. Another machine I tried with NVIDIA graphics seemed not to exhibit the problem. However, it's unclear if this is important or just a red herring.

This RH Bugzilla report has a few more details as well as some xtrace logs from logging into two remote machines, one of which shows the problem and one doesn't:

https://bugzilla.redhat.com/show_bug.cgi?id=1172337