Take default tab in LaunchConfigurationTabGroupViewer from CTabFolder

When setting the active tab of the LaunchConfigurationTabGroupViewer for
the first time (right after setting its input), activate the tab that
the CTabFolder considers as the default tab.

Add a missing test too.

debug/org.eclipse.debug.tests/src/org/eclipse/debug/tests/ui/LaunchConfigurationTabGroupViewerTest.java

@@ -194,6 +194,26 @@ public void testOtherTabIsActivated() {
 		assertThat(((SpyTab) tabs[secondTabIndex])).matches(SpyTab::isActivated, "should have been activated");
 	}
 
+	@Test
+	public void testActivateAllTabs() {
+		ThrowingRunnable<CoreException> activateSeveralTab = () -> {
+			// Create and select launch config
+			fLaunchConfigurationsDialog.getTabViewer().setInput(createLaunchConfigurationInstance());
+
+			// Select all tabs
+			for (int i = 0; i < fLaunchConfigurationsDialog.getTabViewer().getTabs().length; i++) {
+				fLaunchConfigurationsDialog.getTabViewer().setActiveTab(i);
+			}
+		};
+
+		final ILaunchConfigurationTab[] tabs = runOnDialog(activateSeveralTab);
+
+		// All tabs should have been activated exactly once
+		for (int i = 0; i < tabs.length; i++) {
+			assertThat(((SpyTab) tabs[i])).matches(SpyTab::isActivatedExactlyOnce, "Tab '" + i + "' should have been activated exactly once");
+		}
+	}
+
 	private ILaunchConfigurationWorkingCopy createLaunchConfigurationInstance() throws CoreException {
 		return fLaunchConfigurationType.newInstance(null, "MyLaunchConfiguration_" + System.currentTimeMillis());
 	}

debug/org.eclipse.debug.ui/ui/org/eclipse/debug/internal/ui/launchConfigurations/LaunchConfigurationTabGroupViewer.java

@@ -867,15 +867,16 @@ protected void displayInstanceTabs(boolean redrawTabs) {
 		// Update the name field
 		fNameWidget.setText(getWorkingCopy().getName());
 
-		fCurrentTabIndex = fTabFolder.getSelectionIndex();
+		fCurrentTabIndex = -1;
 
 		// Turn off initializing flag to update message
 		fInitializingTabs = false;
 
 		// Try to activate the same (type of) tab that was active before.
 		if (!setActiveTab(lastActiveTabKind)) {
-			// The tab with the wanted class wasn't found. Try to activate the first one
-			setActiveTab(0);
+			// The tab with the wanted class wasn't found. Try to activate the default tab
+			int defaultTabIndex = Math.max(fTabFolder.getSelectionIndex(), 0);
+			setActiveTab(defaultTabIndex);
 		}
 
 		if (!fViewform.isVisible()) {