fix: Add MPR UI id request parameter (#12412)

* fix: Add MPR UI id request parameter

Related-to https://github.com/vaadin/multiplatform-runtime/issues/85

* test: Remove redundant non-empty param test

* test: Remove leftovers

* fix: Init window.mprUiId earlier than window.vaadin

* Add missing '='

server/src/main/resources/VAADIN/vaadinBootstrap.js

@@ -365,6 +365,12 @@
                 params += '&v-wn=' + encodeURIComponent(window.name);
             }
 
+            // This parameter is used in multiplatform-runtime as a key for
+            // storing the MPR UI content in the session
+            if (window.mprUiId) {
+                params += '&v-mui=' + encodeURIComponent(window.mprUiId);
+            }
+
             // Detect touch device support
             var supportsTouch = false;
             try {

uitest/src/main/java/com/vaadin/tests/components/ui/UIInitBrowserDetails.java

@@ -37,6 +37,7 @@ public class UIInitBrowserDetails extends AbstractReindeerTestUI {
         addDetail("dst saving", "v-dstd", wb.getDSTSavings());
         addDetail("dst in effect", "v-dston", wb.isDSTInEffect());
         addDetail("current date", "v-curdate", wb.getCurrentDate());
+        addDetail("mpr ui id", "v-mui", "");
     }
 
     private void addDetail(String name, String param, Object value) {

uitest/src/test/java/com/vaadin/tests/components/ui/UIInitBrowserDetailsTest.java

@@ -11,7 +11,7 @@ import com.vaadin.tests.tb3.MultiBrowserTest;
 public class UIInitBrowserDetailsTest extends MultiBrowserTest {
 
     @Test
-    public void testBrowserDetails() throws Exception {
+    public void testBrowserDetails() {
         openTestURL();
         /* location */
         compareRequestAndBrowserValue("v-loc", "location", "null");
@@ -23,6 +23,9 @@ public class UIInitBrowserDetailsTest extends MultiBrowserTest {
         compareRequestAndBrowserValue("v-sw", "screen width", "-1");
         /* screen height */
         compareRequestAndBrowserValue("v-sh", "screen height", "-1");
+        /* mpr ui id */
+        compareRequestAndBrowserValue("v-mui", "mpr ui id",
+                "any-non-empty-value");
         /* timezone offset */
         assertTextNotNull("timezone offset");
         /* raw timezone offset */