diff --git a/src/main/java/org/antlr/intellij/plugin/parsing/PreviewParser.java b/src/main/java/org/antlr/intellij/plugin/parsing/PreviewParser.java
index 8886531a..f4290efd 100644
--- a/src/main/java/org/antlr/intellij/plugin/parsing/PreviewParser.java
+++ b/src/main/java/org/antlr/intellij/plugin/parsing/PreviewParser.java
@@ -19,6 +19,8 @@ public class PreviewParser extends GrammarParserInterpreter {
 
 	protected int lastSuccessfulMatchState = ATNState.INVALID_STATE_NUMBER; // not sure about error nodes
 
+	private boolean warnedAboutPredicates = false;
+
 	public PreviewParser(Grammar g, ATN atn, TokenStream input) {
 		super(g, atn, input);
 		lexerWatchdog = new LexerWatchdog(input, this);
@@ -33,6 +35,7 @@ public void reset() {
 		super.reset();
 		if ( inputTokenToStateMap!=null ) inputTokenToStateMap.clear();
 		lastSuccessfulMatchState = ATNState.INVALID_STATE_NUMBER;
+		warnedAboutPredicates = false;
 	}
 
 	@Override
@@ -75,4 +78,23 @@ public Token matchWildcard() throws RecognitionException {
 		lastSuccessfulMatchState = getState();
 		return super.matchWildcard();
 	}
+
+	@Override
+	public void action(RuleContext _localctx, int ruleIndex, int actionIndex) {
+		warnAboutPredicatesAndActions();
+	}
+
+	@Override
+	public boolean sempred(RuleContext _localctx, int ruleIndex, int actionIndex) {
+		warnAboutPredicatesAndActions();
+		return super.sempred(_localctx, ruleIndex, actionIndex);
+	}
+
+	private void warnAboutPredicatesAndActions() {
+		if (!warnedAboutPredicates) {
+			notifyErrorListeners("WARNING: predicates and actions are not run by this interpreter. " +
+					"Results may vary from the actual generated parser!");
+			warnedAboutPredicates = true;
+		}
+	}
 }
diff --git a/src/main/java/org/antlr/intellij/plugin/preview/InputPanel.form b/src/main/java/org/antlr/intellij/plugin/preview/InputPanel.form
index 0e977610..3f2f8b43 100644
--- a/src/main/java/org/antlr/intellij/plugin/preview/InputPanel.form
+++ b/src/main/java/org/antlr/intellij/plugin/preview/InputPanel.form
@@ -49,6 +49,16 @@
           </grid>
         </children>
       </grid>
+      <component id="57495" class="com.intellij.openapi.ui.Splitter" binding="editorSplitter">
+        <constraints border-constraint="Center"/>
+        <properties>
+          <dividerWidth value="3"/>
+          <firstComponent value="4d6c1"/>
+          <orientation value="true"/>
+          <proportion value="0.8"/>
+          <secondComponent value="37477"/>
+        </properties>
+      </component>
       <component id="4d6c1" class="javax.swing.JTextArea" binding="placeHolder">
         <constraints border-constraint="East"/>
         <properties>
diff --git a/src/main/java/org/antlr/intellij/plugin/preview/InputPanel.java b/src/main/java/org/antlr/intellij/plugin/preview/InputPanel.java
index 4abe043d..5e642034 100644
--- a/src/main/java/org/antlr/intellij/plugin/preview/InputPanel.java
+++ b/src/main/java/org/antlr/intellij/plugin/preview/InputPanel.java
@@ -17,6 +17,7 @@
 import com.intellij.openapi.fileEditor.FileDocumentManager;
 import com.intellij.openapi.project.Project;
 import com.intellij.openapi.ui.ComponentWithBrowseButton;
+import com.intellij.openapi.ui.Splitter;
 import com.intellij.openapi.ui.TextComponentAccessor;
 import com.intellij.openapi.ui.TextFieldWithBrowseButton;
 import com.intellij.openapi.util.Key;
@@ -70,13 +71,7 @@ public class InputPanel {
 	private JPanel startRuleAndInputPanel;
 	private TextFieldWithBrowseButton fileChooser;
 	private JPanel outerMostPanel;
-
-	/**
-	 * switchToGrammar() was seeing an empty slot instead of a previous
-	 * editor or placeHolder. Figured it was an order of operations thing
-	 * and synchronized add/remove ops. Works now w/o error.
-	 */
-	private final Object swapEditorComponentLock = new Object();
+	private Splitter editorSplitter;
 
 	private final PreviewPanel previewPanel;
 
@@ -266,17 +261,7 @@ public void switchToGrammar(PreviewState previewState, VirtualFile grammarFile)
 	}
 
 	public void setEditorComponent(JComponent editor) {
-		BorderLayout layout = (BorderLayout) outerMostPanel.getLayout();
-		String EDITOR_SPOT_COMPONENT = BorderLayout.CENTER;
-		// atomically remove old
-		synchronized (swapEditorComponentLock) {
-			Component editorSpotComp = layout.getLayoutComponent(EDITOR_SPOT_COMPONENT);
-			if ( editorSpotComp!=null ) {
-				editorSpotComp.setVisible(false);
-				outerMostPanel.remove(editorSpotComp); // remove old editor if it's there
-			}
-			outerMostPanel.add(editor, EDITOR_SPOT_COMPONENT);
-		}
+		editorSplitter.setFirstComponent(editor);
 	}
 
 	public Editor getInputEditor() {