Version: 55.0.2860.0 (Developer Build) (64-bit)
OS: All

What steps will reproduce the problem?
(1) Visit http://flackr.github.io/compositor-worker/parallax-static/parallax.html with the flags --enable-blink-features=GeometryInterfaces,CompositorWorker
(2) Open the inspector and set a breakpoint in parallax.js in the tick function.
(3) Scroll

What is the expected output?
Expect to see the debugger break at the breakpoint.

What do you see instead?
Instead, nothing happens.

This may be related to  issue 646559 . Also, it may be that because we are running on the compositor thread we may not want to actually allow breaking as it may affect other tabs. It is strange though that you can see another thread in the developer tools but cannot debug it.

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