Platform: OS X scrollbars can be disabled for diagnostic purpose

platform/platform-api/src/com/intellij/ui/components/JBScrollPane.java

@@ -122,6 +122,8 @@ public class JBScrollPane extends JScrollPane {
   }
 
   private void transferToLayeredPane(Component old, Component c, String key) {
+    if (!ButtonlessScrollBarUI.isMacOverlayScrollbarSupported()) return;
+    
     JLayeredPane pane = getLayoutPane();
     LayoutManager layout = getLayout();
 
@@ -152,7 +154,9 @@ public class JBScrollPane extends JScrollPane {
   }
   
   private void init(boolean setupCorners) {
-    add(getLayoutPane());
+    if (ButtonlessScrollBarUI.isMacOverlayScrollbarSupported()) {
+      add(getLayoutPane());
+    }
     setLayout(new ScrollPaneLayout());
  
     if (setupCorners) {
@@ -201,7 +205,9 @@ public class JBScrollPane extends JScrollPane {
   @Override
   public void layout() {
     super.layout();
-  
+
+    if (!ButtonlessScrollBarUI.isMacOverlayScrollbarSupported()) return;
+    
     LayoutManager layout = getLayout();
     if (layout instanceof ScrollPaneLayout && myLayeredPane != null) {
       relayoutScrollbars(this, (ScrollPaneLayout)layout, myLayeredPane);

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

@@ -17,6 +17,7 @@ package com.intellij.util.ui;
 
 import com.intellij.openapi.diagnostic.Logger;
 import com.intellij.openapi.util.Disposer;
+import com.intellij.openapi.util.SystemInfo;
 import com.intellij.openapi.util.registry.Registry;
 import com.intellij.ui.Gray;
 import com.intellij.ui.JBColor;
@@ -174,7 +175,11 @@ public class ButtonlessScrollBarUI extends BasicScrollBarUI {
   }
 
   protected boolean isMacOverlayScrollbar() {
-    return myMacScrollerStyle == NSScrollerHelper.Style.Overlay;
+    return myMacScrollerStyle == NSScrollerHelper.Style.Overlay && isMacOverlayScrollbarSupported();
+  }
+  
+  public static boolean isMacOverlayScrollbarSupported() {
+    return SystemInfo.isMac && !Registry.is("ide.mac.disableMacScrollbars");
   }
 
   private void updateMacScrollbarStyle() {

platform/platform-resources-en/src/misc/registry.properties

@@ -142,6 +142,8 @@ ide.mac.hide.cursor.when.typing=true
 ide.mac.show.native.help=true
 ide.mac.useNativeClipboard=false
 ide.mac.boldEditorTabs=false
+ide.mac.disableMacScrollbars=false
+ide.mac.disableMacScrollbars.description=Disables OS X overlay scrollbars (effective on restart)
 ide.perProjectModality=false
 # suppress inspection "UnusedProperty"
 ide.perProjectModality.description=New modality approach. All dialogs are DOCUMENT_MODAL expect ide-wide dialogs