DockableEditorTabbedContainer shouldn't return root pane as accept area as it makes impossible to add any other dockable container. It is better to return root pane in the special fallback method after we checked all containers.

platform/lang-impl/src/com/intellij/execution/ui/layout/impl/RunnerContentUi.java

@@ -449,6 +449,11 @@ public class RunnerContentUi implements ContentUI, Disposable, CellTransform.Fac
     return new RelativeRectangle(myTabs.getComponent());
   }
 
+  @Override
+  public RelativeRectangle getAcceptAreaFallback() {
+    return getAcceptArea();
+  }
+
   @NotNull
   @Override
   public ContentResponse getContentResponse(@NotNull DockableContent content, RelativePoint point) {

platform/lang-impl/src/com/intellij/ide/favoritesTreeView/FavoritesTreeViewPanel.java

@@ -796,6 +796,11 @@ public class FavoritesTreeViewPanel extends JPanel implements DataProvider, Dock
     return new RelativeRectangle(myTree);
   }
 
+  @Override
+  public RelativeRectangle getAcceptAreaFallback() {
+    return getAcceptArea();
+  }
+
   @NotNull
   @Override
   public ContentResponse getContentResponse(@NotNull DockableContent content, RelativePoint point) {

platform/platform-api/src/com/intellij/ui/docking/DockContainer.java

@@ -33,6 +33,11 @@ public interface DockContainer extends Disposable, Activatable {
 
   RelativeRectangle getAcceptArea();
 
+  /**
+   * This area is used when nothing was found with getAcceptArea 
+   */
+  RelativeRectangle getAcceptAreaFallback();
+
   @NotNull
   ContentResponse getContentResponse(@NotNull DockableContent content, RelativePoint point);
 

platform/platform-impl/src/com/intellij/openapi/fileEditor/impl/DockableEditorTabbedContainer.java

@@ -88,6 +88,10 @@ public class DockableEditorTabbedContainer implements DockContainer.Persistent {
 
   @Override
   public RelativeRectangle getAcceptArea() {
+    return new RelativeRectangle(mySplitters);
+  }
+  
+  public RelativeRectangle getAcceptAreaFallback() {
     JRootPane root = mySplitters.getRootPane();
     return root != null ? new RelativeRectangle(root) : new RelativeRectangle(mySplitters);
   }

platform/platform-impl/src/com/intellij/ui/docking/impl/DockManagerImpl.java

@@ -346,6 +346,13 @@ public class DockManagerImpl extends DockManager implements PersistentStateCompo
       }
     }
 
+    for (DockContainer each : myContainers) {
+      RelativeRectangle rec = each.getAcceptAreaFallback();
+      if (rec.contains(point) && each.getContentResponse(content, point).canAccept()) {
+        return each;
+      }
+    }
+
     return null;
   }
 

plugins/terminal/src/org/jetbrains/plugins/terminal/TerminalView.java

@@ -252,27 +252,32 @@ public class TerminalView {
    */
   public class TerminalDockContainer implements DockContainer {
     private ToolWindow myTerminalToolWindow;
-  
+
     public TerminalDockContainer(ToolWindow toolWindow) {
       myTerminalToolWindow = toolWindow;
     }
-  
+
     @Override
     public RelativeRectangle getAcceptArea() {
       return new RelativeRectangle(myTerminalToolWindow.getComponent());
     }
-  
+
+    @Override
+    public RelativeRectangle getAcceptAreaFallback() {
+      return getAcceptArea();
+    }
+
     @NotNull
     @Override
     public ContentResponse getContentResponse(@NotNull DockableContent content, RelativePoint point) {
       return ContentResponse.ACCEPT_MOVE;
     }
-  
+
     @Override
     public JComponent getContainerComponent() {
       return myTerminalToolWindow.getComponent();
     }
-  
+
     @Override
     public void add(@NotNull DockableContent content, RelativePoint dropTarget) {
       if (content.getKey() instanceof TerminalSessionVirtualFileImpl) {
@@ -280,57 +285,57 @@ public class TerminalView {
         myTerminalWidget.addTab(terminalFile.getName(), terminalFile.getTerminal());
       }
     }
-  
+
     @Override
     public void closeAll() {
-  
+
     }
-  
+
     @Override
     public void addListener(Listener listener, Disposable parent) {
-  
+
     }
-  
+
     @Override
     public boolean isEmpty() {
       return false;
     }
-  
+
     @Nullable
     @Override
     public Image startDropOver(@NotNull DockableContent content, RelativePoint point) {
       return null;
     }
-  
+
     @Nullable
     @Override
     public Image processDropOver(@NotNull DockableContent content, RelativePoint point) {
       return null;
     }
-  
+
     @Override
     public void resetDropOver(@NotNull DockableContent content) {
-  
+
     }
-  
+
     @Override
     public boolean isDisposeWhenEmpty() {
       return false;
     }
-  
+
     @Override
     public void showNotify() {
-  
+
     }
-  
+
     @Override
     public void hideNotify() {
-  
+
     }
-  
+
     @Override
     public void dispose() {
-  
+
     }
   }
 }