Move macOS specific behaviour into separate class.

core/src/main/java/com/github/weisj/darklaf/ui/tooltip/DarkMacTooltipUI.java

@@ -0,0 +1,40 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2020 Jannis Weis
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * 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 OR COPYRIGHT HOLDERS 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.
+ *
+ */
+package com.github.weisj.darklaf.ui.tooltip;
+
+import javax.swing.*;
+import javax.swing.plaf.ComponentUI;
+
+public class DarkMacTooltipUI extends DarkToolTipUI {
+
+    public static ComponentUI createUI(final JComponent c) {
+        return new DarkMacTooltipUI();
+    }
+
+    @Override
+    protected boolean useDecoratedPopup() {
+        return true;
+    }
+}

core/src/main/java/com/github/weisj/darklaf/ui/tooltip/DarkToolTipUI.java

@@ -120,7 +120,7 @@ public class DarkToolTipUI extends BasicToolTipUI implements PropertyChangeListe
     @Override
     protected void installDefaults(final JComponent c) {
         super.installDefaults(c);
-        toolTip.putClientProperty(DarkPopupFactory.KEY_NO_DECORATION, true);
+        toolTip.putClientProperty(DarkPopupFactory.KEY_NO_DECORATION, !useDecoratedPopup());
         toolTip.putClientProperty(DarkPopupFactory.KEY_START_HIDDEN, true);
         toolTip.putClientProperty(DarkPopupFactory.KEY_FORCE_HEAVYWEIGHT, true);
         fadeAnimator = new FadeInAnimator();
@@ -128,6 +128,10 @@ public class DarkToolTipUI extends BasicToolTipUI implements PropertyChangeListe
         updateStyle();
     }
 
+    protected boolean useDecoratedPopup() {
+        return false;
+    }
+
     @Override
     public void update(final Graphics g, final JComponent c) {
         // Ensures no background is painted.

core/src/main/resources/com/github/weisj/darklaf/properties/platform/mac.properties

@@ -28,6 +28,8 @@ ScrollBarUI                        = com.github.weisj.darklaf.ui.scrollpane.Dark
 ScrollBar.smallWidth               = 10
 ScrollBar.width                    = 12
 
+TooltipUI                          = com.github.weisj.darklaf.ui.tooltip.DarkMacTooltipUI
+
 Table.alternateRowColor            = true
 Tree.alternateRowColor             = true
 List.alternateRowColor             = true