The scheduler relies on base::ConditionVariable and similar code notifying it when the thread is blocked. WTF::ThreadCondition does not currently report this, which causes the scheduler to not spawn additional worker threads when appropriate, if existing threads are waiting on a ThreadCondition.

This will eventually be resolved by switching to base::Lock/base::ConditionVariable, but in the meantime retrofitting this feature onto the WTF threading primitives is straightforward.