Debugger.NotifyOfCrossThreadDependency can be slow when a debugger is attached

[gregg-miskelly]

Description

Certain code paths will call Debugger.NotifyOfCrossThreadDependency. NotifyOfCrossThreadDependency is pretty close to free when there is no debugger attached, so if you are in a situation where you do something cheap which calls NotifyOfCrossThreadDependency then you can have a very high overhead for the with debugger case compared to the without case. Recently one such case surfaced with Task completion: https://developercommunity.visualstudio.com/content/problem/1097088/attaching-debugger-to-heavily-async-process-causes.html

Ideally, we would avoid notifying the debugger if NotifyOfCrossThreadDependency was called when a func-eval wasn't happening.

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[Mithras]

Here is a working workaround that might be ok for some scenarios (e.g. dev containers): #52149 (comment)