X message deadlocks or similar?

In recent Juce - (problem possible started in code from the last few months), I’ve started to get inter-thread X lock-ups.

The symptom is that my main thread and OpenGL X window threads lock up. Another comms thread with minimal X use seems to run OK. When it’s locked up, I can wiggle the mouse and it seems to unlock and then execute buffered messages (post message).

It doesn’t occur when I remote debug from another machine, so I can’t see where I’m hung up. Nor is there a pattern I can find.

It also only seems to happen on machines with twin graphics cards (2 X screens).

Spec is:

Arch Linux current kernel, no RT additions.
Intel CPU, Nvidia GPUs, 209.10 or so (couldn’t update for a while due to NV bugs)
Current Juce (2 weeks from tip).

Ring a bell for anyone? Or any tips?

Did anything change in the Linux or X messaging code that could have side-effects?