IDEA-149395 Add ability to use background of the scroll pane from the viewport's view

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

@@ -110,8 +110,9 @@ abstract class AbstractScrollBarUI extends ScrollBarUI {
   @Override
   public void installUI(JComponent c) {
     myScrollBar = (JScrollBar)c;
-    myScrollBar.setBackground(new JBColor(new ColorProducer(c, 0xF5F5F5, 0x3C3F41)));
-    myScrollBar.setForeground(new JBColor(new ColorProducer(c, 0xE6E6E6, 0x3C3F41)));
+    boolean auto = Registry.is("ide.scroll.background.auto");
+    myScrollBar.setBackground(auto ? null : new JBColor(new ColorProducer(c, 0xF5F5F5, 0x3C3F41)));
+    myScrollBar.setForeground(auto ? null : new JBColor(new ColorProducer(c, 0xE6E6E6, 0x3C3F41)));
     myScrollBar.setFocusable(false);
     myScrollBar.addMouseListener(myListener);
     myScrollBar.addMouseMotionListener(myListener);

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

@@ -35,6 +35,7 @@ import javax.swing.border.Border;
 import javax.swing.border.LineBorder;
 import javax.swing.plaf.ScrollBarUI;
 import javax.swing.plaf.ScrollPaneUI;
+import javax.swing.plaf.UIResource;
 import javax.swing.plaf.basic.BasicScrollBarUI;
 import java.awt.*;
 import java.awt.event.MouseEvent;
@@ -65,6 +66,7 @@ public class JBScrollPane extends JScrollPane {
 
   private int myViewportBorderWidth = -1;
   private boolean myHasOverlayScrollbars;
+  private volatile boolean myBackgroundRequested; // avoid cyclic references
 
   public JBScrollPane(int viewportWidth) {
     init(false);
@@ -91,6 +93,26 @@ public class JBScrollPane extends JScrollPane {
     init();
   }
 
+  @Override
+  public Color getBackground() {
+    Color color = super.getBackground();
+    if (!myBackgroundRequested && EventQueue.isDispatchThread() && Registry.is("ide.scroll.background.auto")) {
+      if (!isBackgroundSet() || color instanceof UIResource) {
+        Component child = getViewport();
+        if (child != null) {
+          try {
+            myBackgroundRequested = true;
+            return child.getBackground();
+          }
+          finally {
+            myBackgroundRequested = false;
+          }
+        }
+      }
+    }
+    return color;
+  }
+
   public static JScrollPane findScrollPane(Component c) {
     if (c == null) return null;
 

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

@@ -94,6 +94,7 @@ public class JBViewport extends JViewport implements ZoomableViewport {
 
   private Dimension myTempViewSize;
   private boolean mySaveTempViewSize;
+  private volatile boolean myBackgroundRequested; // avoid cyclic references
 
   public JBViewport() {
     addContainerListener(new ContainerListener() {
@@ -117,6 +118,26 @@ public class JBViewport extends JViewport implements ZoomableViewport {
     });
   }
 
+  @Override
+  public Color getBackground() {
+    Color color = super.getBackground();
+    if (!myBackgroundRequested && EventQueue.isDispatchThread() && Registry.is("ide.scroll.background.auto")) {
+      if (!isBackgroundSet() || color instanceof UIResource) {
+        Component child = getView();
+        if (child != null) {
+          try {
+            myBackgroundRequested = true;
+            return child.getBackground();
+          }
+          finally {
+            myBackgroundRequested = false;
+          }
+        }
+      }
+    }
+    return color;
+  }
+
   @Override
   protected LayoutManager createLayoutManager() {
     return ourLayoutManager;

platform/util/resources/misc/registry.properties

@@ -109,6 +109,7 @@ ide.tree.deferred.icon.invalidates.cache=true
 ide.splitter.mouseZone=6
 ide.smart.horizontal.scrolling=true
 ide.scroll.new.layout=true
+ide.scroll.background.auto=false
 ide.scroll.bar.expand.animation=false
 ide.scroll.track.border.paint=false
 ide.scroll.thumb.border.rounded=false