XI1 API may cut XI2 grabs short
Peter Hutterer, Benjamin Tissoires and me have been investigating lately (between others) a bug that would render tablet pad buttons ineffective on X11. Even though Clutter does early passive grabs on those devices, those could sometimes behave as if ungrabbed.
As per Peter's investigation, this is an oversight in the interaction of XI1 and XI2 APIs, in the few places we need touching XI1 API, we do so by opening the device, manipulating it, and closing it immediately. This XCloseDevice call may result in ongoing grabs being cut short, hence the misbehavior.
There's 2 possible fixes we can apply on the mutter side, 1) Using a separate Display for those operations, or 2) making it sure the XI1 XDevice survives long enough.