Fix prune of cached proof branch

Closes #3846

Author: FliegendeWurst

key.core/src/main/java/de/uka/ilkd/key/proof/ProofPruner.java

@@ -7,6 +7,7 @@
 import javax.swing.*;
 
 import de.uka.ilkd.key.proof.init.InitConfig;
+import de.uka.ilkd.key.proof.reference.ClosedBy;
 import de.uka.ilkd.key.rule.NoPosTacletApp;
 import de.uka.ilkd.key.rule.merge.MergePartner;
 import de.uka.ilkd.key.rule.merge.MergeRuleBuiltInRuleApp;
@@ -35,6 +36,16 @@ class ProofPruner {
      * @return the subtrees whose common root was the given {@code cuttingPoint}
      */
     public ImmutableList<Node> prune(final Node cuttingPoint) {
+        // special case: prune cached goal = only need to re-open the final node
+        ClosedBy cby = cuttingPoint.lookup(ClosedBy.class);
+        Goal goal = proof.getClosedGoal(cuttingPoint);
+        if (cby != null && goal != null) {
+            cuttingPoint.deregister(cby, ClosedBy.class);
+            proof.reOpenGoal(goal);
+            refreshGoal(goal, cuttingPoint);
+            return ImmutableList.of(cuttingPoint);
+        }
+
         // there is only one leaf containing an open goal that is interesting for pruning the
         // subtree of <code>node</code>, namely the first leave that is found by a breadth
         // first search.

keyext.caching/src/main/java/de/uka/ilkd/key/gui/plugins/caching/CachingExtension.java

@@ -146,7 +146,7 @@ public void ruleApplied(ProofEvent e) {
         if (!CachingSettingsProvider.getCachingSettings().getEnabled()) {
             return;
         }
-        // new global off switch
+        // new global off switch (toolbar)
         if (!getProofCachingEnabled()) {
             return;
         }