Fix temporary offline of computer is lost on config submit

fixes #26146

Author: mawinter69

core/src/main/java/hudson/model/Computer.java

@@ -1503,7 +1503,9 @@ public void doConfigSubmit(StaplerRequest2 req, StaplerResponse2 rsp) throws IOE
             throw new FormException(Messages.Slave_InvalidConfig_Executors(nodeName), "numExecutors");
         }
 
+        OfflineCause offlineCause = node.getTemporaryOfflineCause();
         Node result = node.reconfigure(req, req.getSubmittedForm());
+        result.setTemporaryOfflineCause(offlineCause);
         Jenkins.get().getNodesObject().replaceNode(this.getNode(), result);
 
         if (result.getNodeProperty(DiskSpaceMonitorNodeProperty.class) != null) {

test/src/test/java/hudson/model/ComputerTest.java

@@ -160,6 +160,18 @@ void computerIconDependsOnOfflineCause() throws Exception {
         assertThat(computer.getIcon(), equalTo(cause.getComputerIcon()));
     }
 
+    @Test
+    @Issue("#26146")
+    void computerTemporaryOfflineCauseStaysOnConfigRound() throws Exception {
+        var agent = j.createSlave();
+        var computer = agent.toComputer();
+        var offlineCause = new OfflineCause.UserCause(User.getOrCreateByIdOrFullName("username"), "Initial cause");
+        j.configRoundtrip(agent);
+        agent = (DumbSlave) j.jenkins.getNode(agent.getNodeName());
+        assertThat(agent.getTemporaryOfflineCause(), equalTo(offlineCause));
+
+    }
+
     @Test
     @LocalData
     void removeUserDetailsFromOfflineCause() throws Exception {