Make getExtendedClientDetails() always return non-null

Browser details are now sent eagerly with the init request, but
getExtendedClientDetails() could still return null before the data
arrives. This changes the API to always return a non-null instance,
using a placeholder with default values (screenWidth = -1, windowName
= null) until actual browser details are available.

For PreserveOnRefresh functionality, the code now checks if windowName
is null rather than checking if the entire ExtendedClientDetails object
is null. This is necessary because:
- Screen dimensions are always sent with the init request
- But window.name may be empty/undefined in new browser windows
- This causes windowName = null even when other details are present
- Different windows with windowName = null would incorrectly share
  cached components

Changes:
- Made ExtendedClientDetails constructor public (marked as internal-only)
- UIInternals.getExtendedClientDetails() creates placeholder if null
- Page.getExtendedClientDetails() simplified to delegate to UIInternals
- Page.retrieveExtendedClientDetails() checks screenWidth == -1
- AbstractNavigationStateRenderer: Changed 3 locations to check
  windowName == null instead of details == null
- WebComponentUI: Changed to check windowName == null

This fixes PreserveOnRefreshIT.refreshInDifferentWindow_componentIsRecreated
test failure where components were incorrectly preserved across different
browser windows due to cache key collisions.

Author: Artur-

flow-server/src/main/java/com/vaadin/flow/component/internal/UIInternals.java

@@ -1347,12 +1347,20 @@ public UI getUI() {
     }
 
     /**
-     * The extended client details, if obtained, are cached in this field.
+     * Returns the extended client details. If browser details have not been
+     * received yet, returns a placeholder instance with default values (all
+     * dimensions set to -1). The placeholder will be updated with actual values
+     * when the browser details are received.
      *
-     * @return the extended client details, or {@literal null} if not yet
-     *         received.
+     * @return the extended client details (never {@code null})
      */
     public ExtendedClientDetails getExtendedClientDetails() {
+        if (extendedClientDetails == null) {
+            // Create placeholder with default values
+            extendedClientDetails = new ExtendedClientDetails(ui, null, null,
+                    null, null, null, null, null, null, null, null, null, null,
+                    null, null, null, null);
+        }
         return extendedClientDetails;
     }
 

flow-server/src/main/java/com/vaadin/flow/component/page/ExtendedClientDetails.java

@@ -101,7 +101,7 @@ public class ExtendedClientDetails implements Serializable {
      * @param navigatorPlatform
      *            navigation platform received from the browser
      */
-    ExtendedClientDetails(UI ui, String screenWidth, String screenHeight,
+    public ExtendedClientDetails(UI ui, String screenWidth, String screenHeight,
             String windowInnerWidth, String windowInnerHeight,
             String bodyClientWidth, String bodyClientHeight, String tzOffset,
             String rawTzOffset, String dstShift, String dstInEffect,

flow-server/src/main/java/com/vaadin/flow/component/page/Page.java

@@ -496,18 +496,7 @@ public interface ExtendedClientDetailsReceiver extends Serializable {
      * @return the extended client details (never {@code null})
      */
     public ExtendedClientDetails getExtendedClientDetails() {
-        ExtendedClientDetails details = ui.getInternals()
-                .getExtendedClientDetails();
-        if (details == null) {
-            // Create placeholder instance with default values
-            ExtendedClientDetails placeholder = new ExtendedClientDetails(ui,
-                    null, null, null, null, null, null, null, null, null, null,
-                    null, null, null, null, null, null);
-            // Store placeholder immediately so we don't return null
-            ui.getInternals().setExtendedClientDetails(placeholder);
-            return placeholder;
-        }
-        return details;
+        return ui.getInternals().getExtendedClientDetails();
     }
 
     /**
@@ -526,12 +515,12 @@ public void retrieveExtendedClientDetails(
             ExtendedClientDetailsReceiver receiver) {
         ExtendedClientDetails details = ui.getInternals()
                 .getExtendedClientDetails();
-        if (details != null && details.getScreenWidth() != -1) {
+        if (details.getScreenWidth() != -1) {
             // Already fetched and complete, call receiver immediately
             receiver.receiveDetails(details);
         } else {
-            // Not available or placeholder, trigger refresh
-            getExtendedClientDetails().refresh(receiver::receiveDetails);
+            // Placeholder with default values, trigger refresh
+            details.refresh(receiver::receiveDetails);
         }
     }
 

flow-server/src/main/java/com/vaadin/flow/component/webcomponent/WebComponentUI.java

@@ -172,16 +172,22 @@ private void connectWebComponent(WebComponentConnectEvent event) {
 
         if (!shouldBePreserved) {
             attachCreatedWebComponent(webComponentConfiguration.get(), event);
-        } else if (getInternals().getExtendedClientDetails() != null) {
-            attachCachedOrCreatedWebComponent(webComponentConfiguration.get(),
-                    event, getComponentHash(event,
-                            getInternals().getExtendedClientDetails()));
         } else {
-            getPage().retrieveExtendedClientDetails(extendedClientDetails -> {
+            ExtendedClientDetails details = getInternals()
+                    .getExtendedClientDetails();
+            if (details.getWindowName() == null) {
+                getPage().retrieveExtendedClientDetails(
+                        extendedClientDetails -> {
+                            attachCachedOrCreatedWebComponent(
+                                    webComponentConfiguration.get(), event,
+                                    getComponentHash(event,
+                                            extendedClientDetails));
+                        });
+            } else {
                 attachCachedOrCreatedWebComponent(
                         webComponentConfiguration.get(), event,
-                        getComponentHash(event, extendedClientDetails));
-            });
+                        getComponentHash(event, details));
+            }
         }
     }
 

flow-server/src/main/java/com/vaadin/flow/router/internal/AbstractNavigationStateRenderer.java

@@ -274,22 +274,22 @@ private boolean populateChain(ArrayList<HasElement> chain,
             if (chain.isEmpty() && isPreservePartialTarget(
                     navigationState.getNavigationTarget(), routeLayoutTypes)) {
                 UI ui = event.getUI();
-                if (ui.getInternals().getExtendedClientDetails() == null) {
+                ExtendedClientDetails details = ui.getInternals()
+                        .getExtendedClientDetails();
+                if (details.getWindowName() == null) {
                     PreservedComponentCache cache = ui.getSession()
                             .getAttribute(PreservedComponentCache.class);
                     if (cache != null && !cache.isEmpty()) {
                         // As there is a cached chain we get the client details
                         // to get the window name so we can determine if the
                         // cache contains a chain for us to use.
                         ui.getPage().retrieveExtendedClientDetails(
-                                details -> handle(event));
+                                d -> handle(event));
                         return true;
                     }
                 } else {
                     Optional<List<HasElement>> partialChain = getWindowPreservedChain(
-                            ui.getSession(),
-                            ui.getInternals().getExtendedClientDetails()
-                                    .getWindowName());
+                            ui.getSession(), details.getWindowName());
                     if (partialChain.isPresent()) {
                         List<HasElement> oldChain = partialChain.get();
                         disconnectElements(oldChain, ui);
@@ -992,18 +992,18 @@ private Optional<ArrayList<HasElement>> getPreservedChain(
         final UI ui = event.getUI();
         final VaadinSession session = ui.getSession();
 
-        if (ui.getInternals().getExtendedClientDetails() == null) {
+        final ExtendedClientDetails details = ui.getInternals()
+                .getExtendedClientDetails();
+        if (details.getWindowName() == null) {
             if (hasPreservedChainOfLocation(session, location)) {
                 // We may have a cached instance for this location, but we
                 // need to retrieve the window name before we can determine
                 // this, so execute a client-side request.
-                ui.getPage().retrieveExtendedClientDetails(
-                        details -> handle(event));
+                ui.getPage().retrieveExtendedClientDetails(d -> handle(event));
                 return Optional.empty();
             }
         } else {
-            final String windowName = ui.getInternals()
-                    .getExtendedClientDetails().getWindowName();
+            final String windowName = details.getWindowName();
             final Optional<ArrayList<HasElement>> maybePreserved = getPreservedChain(
                     session, windowName, event.getLocation());
             if (maybePreserved.isPresent()) {
@@ -1052,7 +1052,7 @@ private void setPreservedChain(ArrayList<HasElement> chain,
         final ExtendedClientDetails extendedClientDetails = ui.getInternals()
                 .getExtendedClientDetails();
 
-        if (extendedClientDetails == null) {
+        if (extendedClientDetails.getWindowName() == null) {
             // We need first to retrieve the window name in order to cache the
             // component chain for later potential refreshes.
             ui.getPage().retrieveExtendedClientDetails(

flow-tests/test-root-context/src/main/java/com/vaadin/flow/uitest/ui/ExtendedClientDetailsView.java

@@ -39,9 +39,9 @@ protected void onShow() {
         // Display initial values immediately
         ExtendedClientDetails details = UI.getCurrentOrThrow().getPage()
                 .getExtendedClientDetails();
-        displayDetails(details, screenWidth, screenHeight,
-                windowInnerWidth, windowInnerHeight, bodyElementWidth,
-                bodyElementHeight, devicePixelRatio, touchDevice);
+        displayDetails(details, screenWidth, screenHeight, windowInnerWidth,
+                windowInnerHeight, bodyElementWidth, bodyElementHeight,
+                devicePixelRatio, touchDevice);
 
         // the sizing values cannot be set with JS but pixel ratio and touch
         // support can be faked
@@ -55,8 +55,8 @@ protected void onShow() {
 
         NativeButton fetchDetailsButton = new NativeButton(
                 "Fetch client details", event -> {
-                    UI.getCurrentOrThrow().getPage()
-                            .getExtendedClientDetails().refresh(updatedDetails -> {
+                    UI.getCurrentOrThrow().getPage().getExtendedClientDetails()
+                            .refresh(updatedDetails -> {
                                 displayDetails(updatedDetails, screenWidth,
                                         screenHeight, windowInnerWidth,
                                         windowInnerHeight, bodyElementWidth,