Offer the switch to the unified diff only when it is enabled

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/contentmergeviewer/TextMergeViewer.java

@@ -4204,11 +4204,12 @@ public void run() {
 	/**
 	 * Adds the action that switches this side by side comparison over to the
 	 * unified diff, the counterpart of the unified diff's action to open the
-	 * comparison. Only added when the input can be shown as a unified diff at all.
+	 * comparison. Only added when the unified diff is enabled in the preferences
+	 * and the input can be shown as one at all.
 	 */
 	private void createShowUnifiedDiffItem(ToolBarManager tbm) {
 		if (!(getCompareConfiguration().getContainer() instanceof CompareEditorInput input)
-				|| !CompareUIPlugin.canShowAsUnifiedDiff(input)) {
+				|| !CompareUIPlugin.canOfferUnifiedDiffSwitch(input)) {
 			return;
 		}
 		Action showUnifiedDiff = new Action() {

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

@@ -897,6 +897,20 @@ public static boolean canShowAsUnifiedDiff(CompareEditorInput input) {
 		return unifiedDiffCandidateOf(input) != null;
 	}
 
+	/**
+	 * Returns whether the classic compare editor may offer a switch to the unified
+	 * diff for the given input. The unified diff is still experimental, so the
+	 * switch stays hidden until the preference that enables it is set.
+	 */
+	public static boolean canOfferUnifiedDiffSwitch(CompareEditorInput input) {
+		if (input == null) {
+			return false;
+		}
+		CompareConfiguration configuration = input.getCompareConfiguration();
+		IPreferenceStore store = configuration == null ? null : configuration.getPreferenceStore();
+		return store != null && store.getBoolean(ComparePreferencePage.UNIFIED_DIFF) && canShowAsUnifiedDiff(input);
+	}
+
 	/**
 	 * Picks the side the unified diff sits on, without reading any content.
 	 * Returns <code>null</code> if neither side qualifies.

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

@@ -57,6 +57,7 @@
 import org.eclipse.core.runtime.IAdaptable;
 import org.eclipse.core.runtime.IPath;
 import org.eclipse.core.runtime.IProgressMonitor;
+import org.eclipse.core.runtime.NullProgressMonitor;
 import org.eclipse.core.runtime.Status;
 import org.eclipse.jface.action.Action;
 import org.eclipse.jface.preference.IPreferenceStore;
@@ -596,6 +597,33 @@ public void testOnlyAQualifyingInputCanBeShownAsUnifiedDiff() throws Exception {
 				"an input without a workspace file must not offer the unified diff"); //$NON-NLS-1$
 	}
 
+	/**
+	 * The unified diff is experimental, so the switch is only offered while the
+	 * preference that enables it is set, and then only for a comparison the unified
+	 * diff can display.
+	 */
+	@Test
+	public void testTheSwitchIsOnlyOfferedWhileTheUnifiedDiffIsEnabled() throws Exception {
+		RecordingCompareEditorInput input = openClassicInput();
+		assertTrue(CompareUIPlugin.canShowAsUnifiedDiff(input),
+				"a workspace file comparison must qualify for the unified diff"); //$NON-NLS-1$
+		assertFalse(CompareUIPlugin.canOfferUnifiedDiffSwitch(input),
+				"a disabled unified diff must not be offered"); //$NON-NLS-1$
+
+		store().setValue(ComparePreferencePage.UNIFIED_DIFF, true);
+		assertTrue(CompareUIPlugin.canOfferUnifiedDiffSwitch(input),
+				"an enabled unified diff must be offered for a qualifying comparison"); //$NON-NLS-1$
+
+		// Prepared without an editor: opening one would leave the fallback to the
+		// classic editor pending past the end of this test.
+		RecordingCompareEditorInput inMemory = new RecordingCompareEditorInput(
+				new InMemoryElement("left.txt", "alpha\nbravo\n"), //$NON-NLS-1$ //$NON-NLS-2$
+				new InMemoryElement("right.txt", "alpha\nBRAVO\n")); //$NON-NLS-1$ //$NON-NLS-2$
+		inMemory.run(new NullProgressMonitor());
+		assertFalse(CompareUIPlugin.canOfferUnifiedDiffSwitch(inMemory),
+				"an enabled unified diff must not be offered for a comparison it cannot display"); //$NON-NLS-1$
+	}
+
 	/**
 	 * Switching a side by side comparison over to the unified diff must leave the
 	 * unified diff behind, not both editors.