Implement preventScroll for focusable elements

appinventor/appengine/src/com/google/appinventor/YaClient.gwt.xml

@@ -103,6 +103,16 @@
     <when-type-assignable class="com.google.gwt.user.client.rpc.RemoteService" />
   </generate-with>
 
+  <replace-with class="com.google.appinventor.client.utils.FocusImplStandard">
+    <when-type-is class="com.google.gwt.user.client.ui.impl.FocusImpl"/>
+    <when-property-is name="user.agent" value="gecko1_8"/>
+  </replace-with>
+
+  <replace-with class="com.google.appinventor.client.utils.FocusImplSafari">
+    <when-type-is class="com.google.gwt.user.client.ui.impl.FocusImpl"/>
+    <when-property-is name="user.agent" value="safari"/>
+  </replace-with>
+
   <!-- English language, independent of country -->
   <extend-property name="locale" values="en"/>
   <!-- Simplified Chinese -->

appinventor/appengine/src/com/google/appinventor/client/utils/FocusImplSafari.java

@@ -0,0 +1,21 @@
+// -*- mode: java; c-basic-offset: 2; -*-
+// Copyright 2020 MIT, All rights reserved
+// Released under the Apache License, Version 2.0
+// http://www.apache.org/licenses/LICENSE-2.0
+
+package com.google.appinventor.client.utils;
+
+import com.google.gwt.dom.client.Element;
+
+/**
+ * FocusImplSafari extends the GWT class of the same name in order to pass
+ * the preventScroll option when an element is focused programmatically.
+ */
+public class FocusImplSafari extends com.google.gwt.user.client.ui.impl.FocusImplSafari {
+  @Override
+  public native void focus(Element elem)/*-{
+    $wnd.setTimeout(function() {
+      elem.focus({'preventScroll': true});
+    }, 0);
+  }-*/;
+}

appinventor/appengine/src/com/google/appinventor/client/utils/FocusImplStandard.java

@@ -0,0 +1,19 @@
+// -*- mode: java; c-basic-offset: 2; -*-
+// Copyright 2020 MIT, All rights reserved
+// Released under the Apache License, Version 2.0
+// http://www.apache.org/licenses/LICENSE-2.0
+
+package com.google.appinventor.client.utils;
+
+import com.google.gwt.dom.client.Element;
+
+/**
+ * FocusImplStandard extends the GWT class of the same name in order to pass
+ * the preventScroll option when an element is focused programmatically.
+ */
+public class FocusImplStandard extends com.google.gwt.user.client.ui.impl.FocusImplStandard {
+  @Override
+  public native void focus(Element elem)/*-{
+    elem.focus({'preventScroll': true});
+  }-*/;
+}