Do not switch to the unified diff after the comparison was closed

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/internal/CompareUIPlugin.java

@@ -957,7 +957,11 @@ protected IStatus run(IProgressMonitor monitor) {
 				}
 				Display.getDefault().asyncExec(() -> {
 					IEditorPart compareEditor = wpage.findEditor(input);
-					if (openUnifiedDiff(source, input, wpage, null, true) && compareEditor != null) {
+					if (compareEditor == null) {
+						// The comparison was closed while it was being prepared.
+						return;
+					}
+					if (openUnifiedDiff(source, input, wpage, null, true)) {
 						// Prompts when the merge has unsaved changes.
 						wpage.closeEditor(compareEditor, true);
 					}

team/tests/org.eclipse.compare.tests/src/org/eclipse/compare/tests/UnifiedDiffOpenTest.java

@@ -58,6 +58,7 @@
 import org.eclipse.core.runtime.IPath;
 import org.eclipse.core.runtime.IProgressMonitor;
 import org.eclipse.core.runtime.Status;
+import org.eclipse.core.runtime.jobs.Job;
 import org.eclipse.jface.action.Action;
 import org.eclipse.jface.preference.IPreferenceStore;
 import org.eclipse.jface.resource.ImageDescriptor;
@@ -621,7 +622,27 @@ public void testSwitchToUnifiedDiffReplacesTheCompareEditor() throws Exception {
 				"switching must reuse the already prepared input"); //$NON-NLS-1$
 	}
 
-	/** A missing resource key would leave the toolbar button blank. */
+	/**
+	 * A comparison the user closed while the switch was being prepared must not
+	 * bring up an editor afterwards.
+	 */
+	@Test
+	public void testSwitchToUnifiedDiffStopsWhenTheComparisonIsClosed() throws Exception {
+		RecordingCompareEditorInput input = openClassicInput();
+
+		CompareUIPlugin.getDefault().switchToUnifiedDiff(input, activePage());
+		// Still on the display thread, so this lands before the switch can open
+		// anything.
+		activePage().closeEditor(activePage().findEditor(input), false);
+
+		pumpUntil(() -> Job.getJobManager().find(input).length == 0, "the switch job did not finish"); //$NON-NLS-1$
+		processQueuedEvents();
+
+		assertEquals(0, activePage().getEditorReferences().length,
+				"no editor may open once the comparison is closed"); //$NON-NLS-1$
+	}
+
+	/** A missing resource key would label the toolbar button with the key itself. */
 	@Test
 	public void testShowUnifiedDiffActionIsFullyDescribed() {
 		ResourceBundle bundle = ResourceBundle.getBundle("org.eclipse.compare.contentmergeviewer.TextMergeViewerResources", //$NON-NLS-1$
@@ -631,7 +652,10 @@ public void testShowUnifiedDiffActionIsFullyDescribed() {
 		};
 		Utilities.initAction(action, bundle, "action.ShowUnifiedDiff."); //$NON-NLS-1$
 
-		assertEquals("Show Unified Diff", action.getText(), "the action needs a label"); //$NON-NLS-1$ //$NON-NLS-2$
+		// A failed lookup yields the key, so the label is checked against that rather
+		// than against the English text, which a translation would replace.
+		assertNotEquals("action.ShowUnifiedDiff.label", action.getText(), "the action needs a label"); //$NON-NLS-1$ //$NON-NLS-2$
+		assertFalse(action.getText().isBlank(), "the action needs a label"); //$NON-NLS-1$
 		assertNotNull(action.getToolTipText(), "the action needs a tooltip"); //$NON-NLS-1$
 		assertNotNull(action.getImageDescriptor(), "the action needs an icon"); //$NON-NLS-1$
 	}