Update:

- Implement PrepareOperations.clearDevModeOverride() method, to clear any
leftover ThreadLocal state, as suggested by Y. Zamani.
- Call PrepareOperations.clearDevModeOverride() in the
StrutsInternalTestCase.setup() to ensure consistent initial state for every
test thread before the tests are executed.
- Call PrepareOperations.clearDevModeOverride() at the end of DebugTagTest
tests that call PrepareOperations.overrideDevMode(), for better state
management in those tests.

core/src/main/java/org/apache/struts2/dispatcher/PrepareOperations.java

@@ -251,4 +251,15 @@ public class PrepareOperations {
         return devModeOverride.get();
     }
 
+    /**
+     * Clear any override of the static devMode value being applied to the current thread.
+     *
+     * This can be useful for any situation where {@link #overrideDevMode(boolean)} might be called
+     * in a flow where {@link #cleanupRequest(javax.servlet.http.HttpServletRequest)} does not get called.
+     * May be very situational (such as some unit tests), but may have other utility as well.
+     */
+    public static void clearDevModeOverride() {
+        devModeOverride.remove();  // Remove current thread's value, enxure next read returns it to initialValue (typically null).
+    }
+
 }

core/src/test/java/org/apache/struts2/StrutsInternalTestCase.java

@@ -21,6 +21,7 @@ package org.apache.struts2;
 import com.opensymphony.xwork2.XWorkTestCase;
 import org.apache.commons.lang3.builder.EqualsBuilder;
 import org.apache.struts2.dispatcher.Dispatcher;
+import org.apache.struts2.dispatcher.PrepareOperations;
 import org.apache.struts2.util.StrutsTestCaseHelper;
 import org.apache.struts2.views.jsp.StrutsMockServletContext;
 import java.util.HashMap;
@@ -37,9 +38,13 @@ public abstract class StrutsInternalTestCase extends XWorkTestCase {
     /**
      * Sets up the configuration settings, XWork configuration, and
      * message resources
+     * 
+     * @throws java.lang.Exception
      */
+    @Override
     protected void setUp() throws Exception {
         super.setUp();
+        PrepareOperations.clearDevModeOverride();  // Clear DevMode override every time (consistent ThreadLocal state for tests).
         initDispatcher(null);
     }
     
@@ -66,6 +71,7 @@ public abstract class StrutsInternalTestCase extends XWorkTestCase {
         return initDispatcher(params);
     }
 
+    @Override
     protected void tearDown() throws Exception {
         super.tearDown();
         // maybe someone else already destroyed Dispatcher

core/src/test/java/org/apache/struts2/views/jsp/ui/DebugTagTest.java

@@ -138,6 +138,8 @@ public class DebugTagTest extends AbstractUITagTest {
         assertTrue("Tag state after doEndTag() under default tag clear state is inequal to new Tag with pageContext/parent set.  " +
                 "May indicate that clearTagStateForTagPoolingServers() calls are not working properly.",
                 strutsBodyTagsAreReflectionEqual(tag, freshTag));
+
+        PrepareOperations.clearDevModeOverride();  // Clear DevMode override. Avoid ThreadLocal side-effects if test thread re-used.
     }
 
     public void testTagAttributeOverrideDevModeTrue_clearTagStateSet() throws Exception {
@@ -159,6 +161,8 @@ public class DebugTagTest extends AbstractUITagTest {
         assertTrue("Tag state after doEndTag() and explicit tag state clearing is inequal to new Tag with pageContext/parent set.  " +
                 "May indicate that clearTagStateForTagPoolingServers() calls are not working properly.",
                 strutsBodyTagsAreReflectionEqual(tag, freshTag));
+
+        PrepareOperations.clearDevModeOverride();  // Clear DevMode override. Avoid ThreadLocal side-effects if test thread re-used.
     }
 
     public void testTagAttributeOverrideDevModeFalse() throws Exception {
@@ -176,6 +180,8 @@ public class DebugTagTest extends AbstractUITagTest {
         assertTrue("Tag state after doEndTag() under default tag clear state is inequal to new Tag with pageContext/parent set.  " +
                 "May indicate that clearTagStateForTagPoolingServers() calls are not working properly.",
                 strutsBodyTagsAreReflectionEqual(tag, freshTag));
+
+        PrepareOperations.clearDevModeOverride();  // Clear DevMode override. Avoid ThreadLocal side-effects if test thread re-used.
     }
 
     public void testTagAttributeOverrideDevModeFalse_clearTagStateSet() throws Exception {
@@ -195,6 +201,8 @@ public class DebugTagTest extends AbstractUITagTest {
         assertTrue("Tag state after doEndTag() and explicit tag state clearing is inequal to new Tag with pageContext/parent set.  " +
                 "May indicate that clearTagStateForTagPoolingServers() calls are not working properly.",
                 strutsBodyTagsAreReflectionEqual(tag, freshTag));
+
+        PrepareOperations.clearDevModeOverride();  // Clear DevMode override. Avoid ThreadLocal side-effects if test thread re-used.
     }
 
     private void setDevMode(final boolean devMode) {