[Edge] Add fallback when WebView2 runtime is not present eclipse-platform#2000

When using WebView2 as browser engine in SWT without a WebView2 runtime
being available on the system, browser initialization fails. In order to
more gracefully handle the case that a system has no such runtime
installed (like on some Windows 10 systems), this change introduces an
automatic fallback option to Internet Explorer in case no WebView2
runtime is found. It shows a dialog informing about the missing runtime,
the option to fallback to IE and also allows to open further information
provided via the FAQ.

Fixes eclipse-platform#2000

Author: HeikoKlare

bundles/org.eclipse.swt/Eclipse SWT Browser/common/org/eclipse/swt/browser/Browser.java

@@ -13,7 +13,10 @@
  *******************************************************************************/
 package org.eclipse.swt.browser;
 
+import java.util.*;
+
 import org.eclipse.swt.*;
+import org.eclipse.swt.program.*;
 import org.eclipse.swt.widgets.*;
 
 /**
@@ -93,10 +96,7 @@ public Browser (Composite parent, int style) {
 	}
 
 	style = getStyle ();
-	webBrowser = new BrowserFactory ().createWebBrowser (style);
-	if (webBrowser != null) {
-		webBrowser.setBrowser (this);
-		webBrowser.create (parent, style);
+	if (createWebBrowser(parent, style)) {
 		return;
 	}
 	dispose ();
@@ -119,6 +119,70 @@ public Browser (Composite parent, int style) {
 	SWT.error (SWT.ERROR_NO_HANDLES, null, errMsg);
 }
 
+private boolean createWebBrowser(Composite parent, int style) {
+	webBrowser = new BrowserFactory ().createWebBrowser (style);
+	if (webBrowser == null) {
+		return false;
+	}
+	webBrowser.setBrowser (this);
+	try {
+		webBrowser.create (parent, style);
+		return true;
+	} catch (SWTError error) {
+		boolean isEdge = "win32".equals(SWT.getPlatform()) && (style & SWT.IE) == 0;
+		if (isEdge && error.code == SWT.ERROR_NOT_IMPLEMENTED) {
+			WebViewUnavailableDialog.showAsync(getShell());
+		}
+		throw error;
+	}
+}
+
+private class WebViewUnavailableDialog {
+	private record DialogOption(int index, String message) {};
+	private static final DialogOption USE_IE_OPTION = new DialogOption(SWT.YES, "Use IE");
+	private static final DialogOption MORE_INFORMATION_OPTION = new DialogOption(SWT.NO, "Information");
+	private static final DialogOption CANCEL_OPTION = new DialogOption(SWT.CANCEL, "Cancel");
+
+	private static final String DIALOG_TITLE = "WebView2 runtime not available";
+	private static final String DIALOG_MESSAGE = "Do you want to use the legacy Internet Explorer engine (requires reopening browsers)?";
+	private static final int DIALOG_OPTION_FLAGS = USE_IE_OPTION.index | MORE_INFORMATION_OPTION.index | CANCEL_OPTION.index;
+	private static final Map<Integer, String> DIALOG_OPTION_LABELS = Map.of( //
+			USE_IE_OPTION.index, USE_IE_OPTION.message, //
+			MORE_INFORMATION_OPTION.index, MORE_INFORMATION_OPTION.message, //
+			CANCEL_OPTION.index, CANCEL_OPTION.message);
+
+	private static boolean shownOnce;
+
+	static void showAsync(Shell parentShell) {
+		if (shownOnce) {
+			return;
+		}
+		shownOnce = true;
+		parentShell.getDisplay().asyncExec(() -> {
+			processDialog(parentShell);
+		});
+	}
+
+	static private void processDialog(Shell parentShell) {
+		MessageBox fallbackInfoBox = new MessageBox(parentShell, SWT.ICON_ERROR | DIALOG_OPTION_FLAGS);
+		fallbackInfoBox.setText(DIALOG_TITLE);
+		fallbackInfoBox.setMessage(DIALOG_MESSAGE);
+		fallbackInfoBox.setButtonLabels(DIALOG_OPTION_LABELS);
+		boolean completed = true;
+		do {
+			int result = fallbackInfoBox.open();
+			if (result == MORE_INFORMATION_OPTION.index) {
+				String faqLink = "https://github.com/eclipse-platform/eclipse.platform/tree/master/docs/FAQ/FAQ_How_do_I_use_Edge-IE_as_the_Browser's_underlying_renderer.md";
+				Program.launch(faqLink);
+				completed = false;
+			} else if (result == USE_IE_OPTION.index) {
+				System.setProperty(PROPERTY_DEFAULTTYPE, "ie");
+				DefaultType = SWT.IE;
+			}
+		} while (!completed);
+	}
+}
+
 static Composite checkParent (Composite parent) {
 	String platform = SWT.getPlatform ();
 	if (!"gtk".equals (platform)) return parent; //$NON-NLS-1$