https://github.com/jenkinsci/workflow-api-plugin/pull/221#discussion_r882853693

Example from `ExecutorStepDynamicContextTest.parallelNodeDisappearance`:

```
"Computer.threadPoolForRemoting [#3]" #89 daemon prio=5 os_prio=0 tid=0x00007f30d0c6a800 nid=0x6d0f9 in Object.wait() [0x00007f31047fe000]
   java.lang.Thread.State: TIMED_WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	at java.lang.Object.wait(Object.java:460)
	at hudson.remoting.AsyncFutureImpl.get(AsyncFutureImpl.java:97)
	- locked <0x00000000f83dd1e8> (a hudson.remoting.AsyncFutureImpl)
	at org.jenkinsci.plugins.workflow.support.steps.ExecutorStepDynamicContext.resume(ExecutorStepDynamicContext.java:108)
	at org.jenkinsci.plugins.workflow.support.steps.ExecutorStepExecution.onResume(ExecutorStepExecution.java:201)
	at org.jenkinsci.plugins.workflow.flow.FlowExecutionList$ParallelResumer.lambda$run$5(FlowExecutionList.java:369)
	at org.jenkinsci.plugins.workflow.flow.FlowExecutionList$ParallelResumer$$Lambda$350/265274739.run(Unknown Source)
	at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)
	at jenkins.security.ImpersonatingExecutorService$1.run(ImpersonatingExecutorService.java:68)
	at …
```

Author: jglick

src/main/java/org/jenkinsci/plugins/workflow/flow/FlowExecutionList.java

@@ -12,6 +12,7 @@
 import hudson.XmlFile;
 import hudson.init.InitMilestone;
 import hudson.init.Terminator;
+import hudson.model.Computer;
 import hudson.model.listeners.ItemListener;
 import hudson.remoting.SingleLaneExecutorService;
 import hudson.util.CopyOnWriteList;
@@ -359,7 +360,10 @@ synchronized void run() {
                 FlowNode n = entry.getKey();
                 StepExecution exec = entry.getValue();
                 processing.add(n);
-                Timer.get().submit(() -> {
+                // Strictly speaking threadPoolForRemoting should be used for agent communications.
+                // In practice the onResume impl known to block is in ExecutorStepExecution.
+                // Avoid jenkins.util.Timer since it is capped at 10 threads and should not be used for long tasks.
+                Computer.threadPoolForRemoting.submit(() -> {
                     LOGGER.fine(() -> "About to resume " + n + " ~ " + exec);
                     try {
                         exec.onResume();