Added GTKLookAndFeelFixer, courtesy Klaus Reimer

https://www.ailis.de/~k/archives/67-Workaround-for-borderless-Java-Swing-menus-on-Linux.html
It makes menu separators visible on linux with the GTK look and feel

arduino-core/src/processing/app/linux/GTKLookAndFeelFixer.java

@@ -0,0 +1,151 @@
+/*
+ * This file is part of Arduino.
+ *
+ * Arduino is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ * As a special exception, you may use this file as part of a free software
+ * library without restriction.  Specifically, if other files instantiate
+ * templates or use macros or inline functions from this file, or you compile
+ * this file and link it with other files to produce an executable, this
+ * file does not by itself cause the resulting executable to be covered by
+ * the GNU General Public License.  This exception does not however
+ * invalidate any other reasons why the executable file might be covered by
+ * the GNU General Public License.
+ *
+ * Copyright 2015 Arduino LLC (http://www.arduino.cc/)
+ */
+
+package processing.app.linux;
+
+import javax.swing.*;
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+
+public class GTKLookAndFeelFixer {
+
+  /*
+   * All functions of this class courtesy of Klaus Reimer
+   * https://www.ailis.de/~k/archives/67-Workaround-for-borderless-Java-Swing-menus-on-Linux.html
+   */
+
+  /*
+   * This is free and unencumbered software released into the public domain.
+   *
+   * Anyone is free to copy, modify, publish, use, compile, sell, or
+   * distribute this software, either in source code form or as a compiled
+   * binary, for any purpose, commercial or non-commercial, and by any
+   * means.
+   *
+   * In jurisdictions that recognize copyright laws, the author or authors
+   * of this software dedicate any and all copyright interest in the
+   * software to the public domain. We make this dedication for the benefit
+   * of the public at large and to the detriment of our heirs and
+   * successors. We intend this dedication to be an overt act of
+   * relinquishment in perpetuity of all present and future rights to this
+   * software under copyright law.
+   *
+   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+   * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+   * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+   * IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+   * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+   * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+   * OTHER DEALINGS IN THE SOFTWARE.
+   *
+   * For more information, please refer to <http://unlicense.org/>
+   */
+
+  /**
+   * Swing menus are looking pretty bad on Linux when the GTK LaF is used (See
+   * bug #6925412). It will most likely never be fixed anytime soon so this
+   * method provides a workaround for it. It uses reflection to change the GTK
+   * style objects of Swing so popup menu borders have a minimum thickness of
+   * 1 and menu separators have a minimum vertical thickness of 1.
+   */
+  public static void installGtkPopupBugWorkaround() {
+    // Get current look-and-feel implementation class
+    LookAndFeel laf = UIManager.getLookAndFeel();
+    Class<?> lafClass = laf.getClass();
+
+    // Do nothing when not using the problematic LaF
+    if (!lafClass.getName().equals("com.sun.java.swing.plaf.gtk.GTKLookAndFeel")) {
+      return;
+    }
+
+    // We do reflection from here on. Failure is silently ignored. The
+    // workaround is simply not installed when something goes wrong here
+    try {
+      // Access the GTK style factory
+      Field field = lafClass.getDeclaredField("styleFactory");
+      boolean accessible = field.isAccessible();
+      field.setAccessible(true);
+      Object styleFactory = field.get(laf);
+      field.setAccessible(accessible);
+
+      // Fix the horizontal and vertical thickness of popup menu style
+      Object style = getGtkStyle(styleFactory, new JPopupMenu(), "POPUP_MENU");
+      fixGtkThickness(style, "yThickness");
+      fixGtkThickness(style, "xThickness");
+
+      // Fix the vertical thickness of the popup menu separator style
+      style = getGtkStyle(styleFactory, new JSeparator(), "POPUP_MENU_SEPARATOR");
+      fixGtkThickness(style, "yThickness");
+    } catch (Exception e) {
+      // Silently ignored. Workaround can't be applied.
+    }
+  }
+
+  /**
+   * Called internally by installGtkPopupBugWorkaround to fix the thickness
+   * of a GTK style field by setting it to a minimum value of 1.
+   *
+   * @param style     The GTK style object.
+   * @param fieldName The field name.
+   * @throws Exception When reflection fails.
+   */
+  private static void fixGtkThickness(Object style, String fieldName) throws Exception {
+    Field field = style.getClass().getDeclaredField(fieldName);
+    boolean accessible = field.isAccessible();
+    field.setAccessible(true);
+    field.setInt(style, Math.max(1, field.getInt(style)));
+    field.setAccessible(accessible);
+  }
+
+  /**
+   * Called internally by installGtkPopupBugWorkaround. Returns a specific
+   * GTK style object.
+   *
+   * @param styleFactory The GTK style factory.
+   * @param component    The target component of the style.
+   * @param regionName   The name of the target region of the style.
+   * @return The GTK style.
+   * @throws Exception When reflection fails.
+   */
+  private static Object getGtkStyle(Object styleFactory, JComponent component, String regionName) throws Exception {
+    // Create the region object
+    Class<?> regionClass = Class.forName("javax.swing.plaf.synth.Region");
+    Field field = regionClass.getField(regionName);
+    Object region = field.get(regionClass);
+
+    // Get and return the style
+    Class<?> styleFactoryClass = styleFactory.getClass();
+    Method method = styleFactoryClass.getMethod("getStyle", JComponent.class, regionClass);
+    boolean accessible = method.isAccessible();
+    method.setAccessible(true);
+    Object style = method.invoke(styleFactory, component, region);
+    method.setAccessible(accessible);
+    return style;
+  }
+}

arduino-core/src/processing/app/linux/Platform.java

@@ -29,7 +29,9 @@ import processing.app.debug.TargetPackage;
 import processing.app.legacy.PConstants;
 import processing.app.tools.CollectStdOutExecutor;
 
-import java.io.*;
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.IOException;
 import java.util.Map;
 
 
@@ -42,17 +44,7 @@ public class Platform extends processing.app.Platform {
   // TODO Need to be smarter here since KDE people ain't gonna like that GTK.
   //      It may even throw a weird exception at 'em for their trouble.
   public void setLookAndFeel() throws Exception {
-    // Linux is by default even uglier than metal (Motif?).
+    GTKLookAndFeelFixer.installGtkPopupBugWorkaround();
-    // Actually, i'm using native menus, so they're even uglier
-    // and Motif-looking (Lesstif?). Ick. Need to fix this.
-    //String lfname = UIManager.getCrossPlatformLookAndFeelClassName();
-    //UIManager.setLookAndFeel(lfname);
-
-    // For 0120, trying out the gtk+ look and feel as the default.
-    // This is available in Java 1.4.2 and later, and it can't possibly
-    // be any worse than Metal. (Ocean might also work, but that's for
-    // Java 1.5, and we aren't going there yet)
-    //UIManager.setLookAndFeel("com.sun.java.swing.plaf.gtk.GTKLookAndFeel");
   }