displaying mnemonics fixed for IdeaTitledBorder

(actually they do not work here) (cherry picked from commit 591c2cf)

platform/platform-api/src/com/intellij/ui/border/IdeaTitledBorder.java

@@ -1,6 +1,7 @@
 package com.intellij.ui.border;
 
 import com.intellij.ui.TitledSeparator;
+import com.intellij.util.ui.DialogUtil;
 import com.intellij.util.ui.UIUtil;
 
 import javax.swing.*;
@@ -20,19 +21,25 @@ public class IdeaTitledBorder extends TitledBorder {
     super(title);
     titledSeparator = new TitledSeparator(title);
     titledSeparator.setText(title);
+    DialogUtil.registerMnemonic(titledSeparator.getLabel(), null);
 
     this.outsideInsets = new Insets(insets.top, insets.left, insets.bottom, insets.right);
 
     this.insideInsets = new Insets(TitledSeparator.BOTTOM_INSET, indent, 0, 0);
   }
 
+  @Override
+  public void setTitle(String title) {
+    super.setTitle(title);
+    titledSeparator.setText(title);
+  }
+
   @Override
   public void paintBorder(Component c, Graphics g, int x, int y, int width, int height) {
     int labelX = x + outsideInsets.left;
     int labelY = y + outsideInsets.top;
 
     TitledSeparator titledSeparator = getTitledSeparator(c);
-    titledSeparator.setText(title);
     JLabel label = titledSeparator.getLabel();
     Dimension labelSize = label.getPreferredSize();
     label.setSize(labelSize);

platform/platform-api/src/com/intellij/util/ui/DialogUtil.java

@@ -16,6 +16,7 @@
 package com.intellij.util.ui;
 
 import com.intellij.ide.ui.UISettings;
+import org.jetbrains.annotations.Nullable;
 
 import javax.swing.*;
 
@@ -66,7 +67,7 @@ public class DialogUtil{
     }
   }
 
-  public static void registerMnemonic(JLabel label, JComponent target) {
+  public static void registerMnemonic(JLabel label, @Nullable JComponent target) {
     registerMnemonic(label, target, UIUtil.MNEMONIC);
   }
 
@@ -76,7 +77,7 @@ public class DialogUtil{
    * @param mn mnemonic char
    * @since 11.0
    */
-  public static void registerMnemonic(JLabel label, JComponent target, char mn) {
+  public static void registerMnemonic(JLabel label, @Nullable JComponent target, char mn) {
     String text = label.getText();
     if (text != null) {
       final StringBuilder realText = new StringBuilder(text.length());