Before mesa 18.0 glXWaitVideoSyncSGI worked perfectly, but after updating to mesa 18 it blocks forever, see the following for the code exhibiting this behavior.
We have seen this occur on AMD hardware, I personally can reproduce it on a Vega 56. It may affect other platforms but I can not confirm.
Please attach a backtrace of the app when it hangs, and the corresponding Xorg log file.
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