Don't remove parts from part stack on DnD drop

Currently when dropping elements onto a part stack MStackElements are removed if their element ids are equal.
This leads to disappearing parts in case of
- at least one part created using part descriptors allowing multiple instances is present in the part stack
- another such part is dropped onto the part stack

Following the comments/notes the intention is to remove place holder parts with the same element id as the
element's id being dropped.

This fix ensures that only instances of type MPlaceholder are removed from the part stack.

Fixes #2771

Author: p-O-q

bundles/org.eclipse.e4.ui.workbench.addons.swt/META-INF/MANIFEST.MF

@@ -1,7 +1,7 @@
 Manifest-Version: 1.0
 Bundle-ManifestVersion: 2
 Bundle-SymbolicName: org.eclipse.e4.ui.workbench.addons.swt;singleton:=true
-Bundle-Version: 1.5.600.qualifier
+Bundle-Version: 1.5.700.qualifier
 Bundle-Name: %pluginName
 Bundle-Vendor: %providerName
 Bundle-Localization: plugin

bundles/org.eclipse.e4.ui.workbench.addons.swt/src/org/eclipse/e4/ui/workbench/addons/dndaddon/StackDropAgent.java

@@ -11,6 +11,7 @@
  * Contributors:
  *     IBM Corporation - initial API and implementation
  *     Patrik Suzzi <[email protected]> - Bug 497348
+ *     Oliver Lins <[email protected]> - Issue 2771
  ******************************************************************************/
 
 package org.eclipse.e4.ui.workbench.addons.dndaddon;
@@ -21,6 +22,7 @@
 import java.util.stream.Stream;
 import org.eclipse.e4.ui.internal.workbench.swt.AbstractPartRenderer;
 import org.eclipse.e4.ui.model.application.ui.MUIElement;
+import org.eclipse.e4.ui.model.application.ui.advanced.MPlaceholder;
 import org.eclipse.e4.ui.model.application.ui.basic.MPartStack;
 import org.eclipse.e4.ui.model.application.ui.basic.MStackElement;
 import org.eclipse.e4.ui.model.application.ui.basic.MWindow;
@@ -260,13 +262,19 @@ private void dock(MUIElement dragElement, int dropIndex) {
 		// Note 3: currently if we drag/drop parts, it looks like for editor parts we
 		// always drop PartImpl instances, for views we drop PartStackImpl or
 		// PlaceholderImpl instances. So one could use this for the check below too.
+		// Note 4: b/c of parts allowing multiple instances with the same element id
+		// it is necessary to also test for the instance type of the children of the
+		// drop stack. If an instance is of type MPlaceholder and the element id is
+		// equal to the drag element's element id the placeholder is removed.
 		MStackElement viewWithSameId = null;
 		if (elementIndex == -1 && !dragElement.getTags().contains("Editor")) { //$NON-NLS-1$
 			for (MStackElement stackElement : dropChildren) {
-				String id = stackElement.getElementId();
-				if (id != null && id.equals(dragElement.getElementId())) {
-					viewWithSameId = stackElement;
-					break;
+				if (stackElement instanceof MPlaceholder placeholder) {
+					String id = placeholder.getElementId();
+					if (id != null && id.equals(dragElement.getElementId())) {
+						viewWithSameId = placeholder;
+						break;
+					}
 				}
 			}
 		}

tests/org.eclipse.e4.ui.workbench.addons.swt.test/META-INF/MANIFEST.MF

@@ -2,15 +2,16 @@ Manifest-Version: 1.0
 Bundle-ManifestVersion: 2
 Bundle-Name: %pluginName
 Bundle-SymbolicName: org.eclipse.e4.ui.workbench.addons.swt.test
-Bundle-Version: 1.5.0.qualifier
+Bundle-Version: 1.5.100.qualifier
 Bundle-Localization: plugin
-Fragment-Host: org.eclipse.e4.ui.workbench.addons.swt;bundle-version="[1.1.0,2.0.0)"
+Fragment-Host: org.eclipse.e4.ui.workbench.addons.swt;bundle-version="[1.5.700,2.0.0)"
 Bundle-RequiredExecutionEnvironment: JavaSE-17
 Require-Bundle: org.junit;bundle-version="[4.12.0,5.0.0)",
  org.eclipse.e4.core.commands;bundle-version="[1.0.0,2.0.0)",
  org.eclipse.core.databinding.observable;bundle-version="[1.4.0,2.0.0)",
  org.eclipse.jface.databinding;bundle-version="[1.6.200,2.0.0)",
- org.eclipse.e4.ui.workbench.swt;bundle-version="[0.12.100,1.0.0)"
+ org.eclipse.e4.ui.workbench.swt;bundle-version="[0.12.100,1.0.0)",
+ org.mockito.mockito-core;bundle-version="5.15.2"
 Bundle-Vendor: %providerName
 Export-Package: org.eclipse.e4.ui.workbench.addons.minmax;x-internal:=true
 Automatic-Module-Name: org.eclipse.e4.ui.workbench.addons.swt.test

tests/org.eclipse.e4.ui.workbench.addons.swt.test/src/org/eclipse/e4/ui/workbench/addons/AllTests.java

@@ -1,6 +1,7 @@
 package org.eclipse.e4.ui.workbench.addons;
 
 import org.eclipse.e4.ui.workbench.addons.cleanupaddon.CleanupAddonTest;
+import org.eclipse.e4.ui.workbench.addons.dndaddon.StackDropAgentTest;
 import org.eclipse.e4.ui.workbench.addons.minmax.MaximizableChildrenTag;
 import org.eclipse.e4.ui.workbench.addons.minmax.MaximizeBugTest;
 import org.eclipse.e4.ui.workbench.addons.minmax.MaximizePartSashContainerPlaceholderTest;
@@ -14,6 +15,7 @@
 		MaximizePartSashContainerPlaceholderTest.class, //
 		MaximizableChildrenTag.class, //
 		CleanupAddonTest.class, //
+		StackDropAgentTest.class, //
 })
 public class AllTests {