To reproduce, run the HelloAnimation test program from apps/toys/Hello on the metal pipeline. It will run for a while and then the rendering will stop. It actually isn't completely hung, but it looks like it is. There is no more rendering at all, and the debug output to the console slows to one "frame" every 30 second to 1 minute (with no visible change to the window).

Here is the stack trace of the QuantumRenderer Thread most of the time when I check:

"QuantumRenderer-0" #14 daemon prio=5 os_prio=31 cpu=5597.46ms elapsed=31.40s tid=0x00007f95c6817200 nid=0x9e03 runnable [0x000070000db9d000]
   java.lang.Thread.State: RUNNABLE
at com.sun.prism.mtl.MTLRTTexture.nReadPixelsFromContextRTT(javafx.graphics@18-internal/Native Method)
at com.sun.prism.mtl.MTLRTTexture.getPixels(javafx.graphics@18-internal/MTLRTTexture.java:137)
at com.sun.javafx.tk.quantum.UploadingPainter.run(javafx.graphics@18-internal/UploadingPainter.java:178)