This has the same root cause as <https://webkit.org/b/135830>. Here, CVDisplayLink gets frame draw intervals from Quartz, then sends that to ScriptedAnimationController via callOnMainThread to signal it's time to service callbacks. But, callOnMainThread dispatching is stuck for all pages while the debugger is paused and running a nested event loop. The best fix is moving inspector out-of-process.
This has been made unnecessary by the fix to https://bugs.webkit.org/show_bug.cgi?id=135120