Support Reuse of Last Opened Editor when JDT Plugin is present

Adds `OpenFileWithReuseActionProvider` that wraps the current OPEN
handler (dependsOn org.eclipse.jdt.ui.navigator.actions.OpenActions) and
installs a wrapper OPEN action in Project Explorer.
For single IFile selections (pref enabled), reuse the last editor via
`OpenFileWithReuseAction`. For all other selections (e.g.,
IJavaElement), delegate to the previously installed OPEN handler (JDT's
OpenAction), preserving Java behavior.

Author: sebthom

bundles/org.eclipse.ui.navigator.resources/plugin.xml

@@ -233,6 +233,17 @@
            </enablement>
         </actionProvider>
 
+        <!-- Wrapper OPEN handler for Project Explorer: applies reuse-last-open-editor for IFile, delegates otherwise -->
+        <actionProvider
+              class="org.eclipse.ui.internal.navigator.resources.actions.OpenFileWithReuseActionProvider"
+              id="org.eclipse.ui.navigator.resources.ReuseOpenActions"
+              dependsOn="org.eclipse.jdt.ui.navigator.actions.OpenActions">
+           <!-- true: always enabled so we can wrap any OPEN handler and delegate when needed -->
+           <enablement>
+              <or/>
+           </enablement>
+        </actionProvider>
+
         <actionProvider
               class="org.eclipse.ui.internal.navigator.resources.actions.ResourceMgmtActionProvider"
               id="org.eclipse.ui.navigator.resources.ResourceMgmtActions">

bundles/org.eclipse.ui.navigator.resources/src/org/eclipse/ui/internal/navigator/resources/actions/OpenFileWithReuseActionProvider.java

@@ -0,0 +1,121 @@
+/*******************************************************************************
+ * Copyright (c) 2025 Vegard IT GmbH and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *   Sebastian Thomschke (Vegard IT GmbH) - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.ui.internal.navigator.resources.actions;
+
+import org.eclipse.core.resources.IFile;
+import org.eclipse.core.runtime.Adapters;
+import org.eclipse.jface.action.Action;
+import org.eclipse.jface.action.IAction;
+import org.eclipse.jface.viewers.ISelection;
+import org.eclipse.jface.viewers.IStructuredSelection;
+import org.eclipse.jface.viewers.StructuredSelection;
+import org.eclipse.ui.IActionBars;
+import org.eclipse.ui.IPageLayout;
+import org.eclipse.ui.IWorkbenchPage;
+import org.eclipse.ui.actions.ActionContext;
+import org.eclipse.ui.actions.OpenFileWithReuseAction;
+import org.eclipse.ui.navigator.CommonActionProvider;
+import org.eclipse.ui.navigator.ICommonActionConstants;
+import org.eclipse.ui.navigator.ICommonActionExtensionSite;
+import org.eclipse.ui.navigator.ICommonViewerWorkbenchSite;
+
+/**
+ * Installs a wrapper OPEN handler in Project Explorer that applies last opened
+ * editor reuse for single {@link IFile} selections, and delegates to the
+ * previous OPEN handler otherwise (e.g., JDT OpenAction for Java elements).
+ */
+public class OpenFileWithReuseActionProvider extends CommonActionProvider {
+
+	private ICommonViewerWorkbenchSite viewSite;
+	private OpenFileWithReuseAction reuseAction;
+	private IWorkbenchPage reusePage;
+
+	@Override
+	public void init(ICommonActionExtensionSite aConfig) {
+		if (aConfig.getViewSite() instanceof ICommonViewerWorkbenchSite) {
+			viewSite = (ICommonViewerWorkbenchSite) aConfig.getViewSite();
+		}
+	}
+
+	@Override
+	public void fillActionBars(IActionBars actionBars) {
+		if (viewSite == null) {
+			return;
+		}
+		// Scope strictly to Project Explorer (including secondary ids)
+		String partId = viewSite.getSite().getId();
+		if (partId == null || !partId.startsWith(IPageLayout.ID_PROJECT_EXPLORER)) {
+			return;
+		}
+
+		// Capture current OPEN delegate (e.g., JDT's OpenAction) to call when reuse
+		// doesn't apply
+		IAction delegate = actionBars.getGlobalActionHandler(ICommonActionConstants.OPEN);
+
+		IAction wrapper = new Action() {
+			@Override
+			public void run() {
+				ISelection sel = getSelection();
+				if (!(sel instanceof IStructuredSelection)) {
+					if (delegate != null) {
+						delegate.run();
+					}
+					return;
+				}
+				IStructuredSelection ss = (IStructuredSelection) sel;
+				if (ss.size() == 1) {
+					Object element = ss.getFirstElement();
+					IFile file = Adapters.adapt(element, IFile.class);
+					if (file != null) {
+						IWorkbenchPage page = viewSite.getPage();
+						if (reuseAction == null || reusePage != page) {
+							reuseAction = new OpenFileWithReuseAction(page, IPageLayout.ID_PROJECT_EXPLORER);
+							reusePage = page;
+						}
+						reuseAction.selectionChanged(new StructuredSelection(file));
+						reuseAction.run();
+						return;
+					}
+				}
+				// Fallback: delegate to prior handler (e.g., JDT)
+				if (delegate != null) {
+					delegate.run();
+				}
+			}
+
+			@Override
+			public void setEnabled(boolean enabled) {
+				super.setEnabled(enabled);
+				if (delegate != null) {
+					delegate.setEnabled(enabled);
+				}
+			}
+		};
+
+		// Install synchronously; plugin.xml dependsOn ensures we run after JDT's
+		// provider
+		actionBars.setGlobalActionHandler(ICommonActionConstants.OPEN, wrapper);
+		actionBars.updateActionBars();
+	}
+
+	private ISelection getSelection() {
+		ActionContext ctx = getContext();
+		if (ctx != null && ctx.getSelection() != null) {
+			return ctx.getSelection();
+		}
+		return viewSite.getSelectionProvider() != null //
+				? viewSite.getSelectionProvider().getSelection()
+				: StructuredSelection.EMPTY;
+	}
+}